Young, brash, and conservative.

When William F. Buckley Jr. was fresh out of Yale, he wrote a book that rocked his alma mater. As a young man he expressed his patriotism by working for the Central Intelligence Agency and then starting his own conservative journal of opinion (National Review).

Thanks to his brash, combative style, the flavor of conservatism has never been the same since.

If he were graduating today, Mr. Buckley just might join the Heritage Foundation.

Thanks to this foundation, Washington think tanks - conservative or liberal - will never be quite the same again, either.

Heritage hit the policy beaches of Washington 11 years ago, the intellectual equivalent of the US Marines. Provocative as a commando raid in broad daylight, it seized an ideological beachhead and started making the case for slashing government spending, cutting regulations, and rebuilding the military.

Its goal has been not so much to think about conservative policies as to help turn them into marching orders - a goal that sets Heritage starkly apart from the less advocacy-oriented think tanks.

Also setting Heritage apart is the tone of its approach: a certain self-assured bravado, a confidence that it can make a difference in the marketplace of ideas and that it can get the government to buy its policies, not sometime in the future, but right now.

A stream of policy papers, books, monographs, newsletters, and reviews on domestic and foreign policy issues pours from its offices - on average 60 to 100 of them annually. Some are written in-house, others by outside scholars commissioned by Heritage. All are geared to exploit conservative policy opportunities whenever possible. To this end they are often hand-delivered to nearly 1,000 policymakers in Congress and the administration. In addition, they are mailed out to some 6,000 journalists, editors, academics, and contributors.

True to its ''travel light and fight hard'' philosophy, more than 70 percent of these publications must pass what Herbert B. Berkowitz, vice-president of Heritage, calls the briefcase test:

''Can it be read,'' he asks, ''in the back seat of a limousine on the way to giving testimony before some congressional committee in the time it takes (approximately 20 minutes) to drive from National Airport to Capitol Hill?'' If not, ''then probably the American Enterprise Institute should write it,'' he says with a chuckle.

''We want to see conservative scholarship have an impact on Washington,'' Mr. Berkowitz says. ''People on college faculties write the books. We read, digest, synthesize, and adapt them to the real world as public policy.''

In 1979, in keeping with its aggressive style, Heritage's leadership correctly assessed the chances a conservative Republican would have to be elected president in 1980. And the foundation was ready with a detailed, nitty-gritty, sleeves-rolled-up study of how to effect policy change in Washington. The report, ''Mandate for Leadership'' - all 1,093 pages of it, was published soon after Ronald Reagan entered the White House.

President Reagan has praised the foundation on several occasions, referring to it as ''that feisty new kid on the conservative block'' and crediting Heritage as a source of the ideas used in his administration's policymaking process. In the world of think tanks, such an endorsement is almost better than a blank check from the US Treasury.

The rise of Heritage to this level of influence has been nothing less than meteoric - especially considering that, unlike other think tanks, which employ senior policymakers and veteran strategists, its staff is almost totally composed of people under age 40. But access to power has in no way mellowed its brashness. Heritage gave the White House only a 62 percent ''compliance rating'' on major conservative issues at the end of Mr. Reagan's first 12 months in office.

Foundation president Edwin J. Fuelner Jr. received a ''Dear Ed'' letter from President Reagan saying that the criticism was well taken and that ''... I am grateful for your dedication and am looking forward to working with you and the members of this team in pursuit of our common goals for a better America.''

Heritage's financial fortunes have also risen sharply during the unabashedly patriotic Reagan years. Founded in 1973 with $250,000 from Colorado brewer Joseph Coors, Heritage could list revenues at a little more than $1 million by 1975. Money flowing into the coffers has gone up 40 percent a year since 1976, totaling $10 million for 1983. That year Heritage moved its headquarters from a suite of offices in a run-down, out-of-the-way building in northeastern Washington (it once housed a grocery store and a halfway house for drug addicts) to a new, eight-story, $9.5 million office building on Capitol Hill.

In addition to its in-house staff, Heritage taps a network of conservatives at some 450 research groups, as well as about 1,100 scholars and public-policy experts in universities. It helps bring a number of these individuals to Washington to testify as expert witnesses at congressional and administrative agency hearings.

A former Heritage fellow, along with a scholar who was a contract researcher for Heritage, now work for the influential editorial page of the Wall Street Journal. And Adam Meyerson, formerly on the Journal's editorial board, now edits Heritage's Policy Review magazine.

The 35 resident scholars and 75 supporting staff take a free-market approach to domestic and economic policy studies. On foreign policy and defense, Heritage takes a strong pro-national-security position.

Moreover, Heritage, unlike such gloomy prognosticators as the Club of Rome, believes in a future that works. One of its more controversial studies predicts less crowding (despite continued population growth), less pollution, and fewer people living at the economic margin by the year 2000 than today.

In the area of civil rights, Heritage is strongly opposed to racial quotas. Its publication ''Agenda for '83'' seeks to redefine ''discrimination.'' Position papers argue that the Justice Department should switch from civil rights lawsuits to prosecuting affirmative-action violations based on quotas. The think tank seems to have had an impact in this area, to judge from Reagan administration policies on this issue and the kind of suits filed recently by the Justice Department.

On the Mideast, Heritage is pro-Israel. In fact, an Israeli general is here as a visiting fellow. ''No other think tank has someone like that on their staff ,'' says Mr. Berkowitz. ''We thought looking at the Middle East the way a military commander does would be helpful.'' Officers from the US military also serve as visiting fellows at Heritage, a practice also common at other think tanks.

One of Heritage's hardest-hitting publications, ''A World Without the UN,'' characterizes the international organization as a forum for attacks on Western interests and democratic values and suggests that it makes world conflicts worse rather than helping to resolve them. The paper calls for US withdrawal from the organization unless broad changes are made. ''Far from cooling passions,'' the study states of anti-Western rhetoric, ''the techniques of name-calling and lying are intended to mobilize the Assembly on the side of the speaker, to discredit and isolate adversaries, and to cultivate climates of opinion inhospitable to rational argument.''

If Walter Mondale is searching for the ''beef'' in Ronald Reagan's program for a second administration, the Heritage Foundation would be a good place to start. ''Mandate for Leadership II,'' just as detailed and specific as its predecessor, is in the final stages now and will be released shortly after the November election.