For lease: inside guide to the Pentagon. Defense consultants can earn more than $1,000 a day selling their knowledge of the Pentagon's corridors of power. A slowdown in spending for weapons systems has only increased the value of these `guides' as competition grows to win contracts.

Some are called rent-a-generals, though their ranks include former admirals, colonels, and those who have never worn uniforms. They act as the Lewis and Clarks of the military-industrial complex, guiding their clients, often defense contractors, through the thicket of rules and regulations, wants and wishes of the Pentagon.

Now, however, defense consultants are coming under increasing scrutiny as a result of the federal probe into alleged fraud and bribery in weapons buying.

As congressional inquiries into the military procurement process mount, those who advise the Pentagon, defense contractors - and sometimes both at once - are emerging as a central focus of concern.

The question: Has the relationship between consultants and their military and industrial clients become too incestuous and are more controls needed?

A Senate governmental affairs subcommittee took up the issue today. The House Armed Services Committee touched on it earlier this week.

``There are just huge problems in this whole business,'' says Sen. David Pryor (D) of Arkansas, chairman of the Federal Services subcommittee.

No one knows for sure how many defense consultants there are, in part because of the differing definitions of what one is. But there seems to be no shortage of them: The Pentagon will spend $2.2 billion on consultants this year.

That number doesn't include the hundreds who work for defense contractors, many of whom inhabit the smoked-glass office buildings that fringe Washington and are part of the army of government consultants nicknamed Beltway bandits. Their ranks swelled in the early years of the Reagan military buildup.

Now, even though the amount of money being put into weapons systems has slowed, many are still in demand. The growing size and complexity of the procurement process is increasingly requiring the skills of specialists. With fewer contracts being let, competition among contractors has also increased, leading some to place more reliance on consultants who can give them an edge in the bidding process.

``Knowledge is power in winning programs,'' says one industry official whose company uses defense consultants.

Many of the former military brass, congressional aides, or government employees, defense consultants play different roles: from management consultant to lobbyist to technical expert. Often, though, they are most coveted for divining what the military wants and understanding the weapons-buying process. Those who are ex-military - some of whom charge well more than $1,000 a day for their services - have value beyond that.

``They are often being bought for their personal contacts and influence,'' says Richard Stubbing, a professor at Duke University and a former defense specialist at the Office of Management and Budget.

It is the growing pressure for inside information - and the money that can be gotten for it - that some experts speculate lies behind any trafficking in confidential government information that may have occurred. Defense consultants maintain that most members of their fraternity are honorable and ethical. But enough have been implicated in the current weapons-buying probe that some lawmakers want closer monitoring of their activities.

A chief concern is consultants who advise both the Defense Department and industry. Pentagon officials say consultants are usually not under direct contract to both at the same time. But the overlapping often happens in more oblique ways: A consultant will be advising the Pentagon as part of his duties on a defense advisory board, while taking industry clients.

Some consultants defend this dual role as enhancing communication between the military and weapons makers. But others see it leading to conflicts of interest. They argue that it can become too tempting for consultants to pass inside information gleaned at the Pentagon on to private contractors or other friends and consultants.

At a House Armed Services Committee hearing this week, Derek Vander Schaaf, the Pentagon's deputy inspector general, described such relationships as ``incestuous'' and said the situation had ``gotten out of hand.'' He noted that information generated by the Defense Department's consultants ``has a tendency to leak out into the marketplace.''

Some fault the Pentagon for not doing enough to spot such abuses. Les Aspin (D) of Wisconsin, chairman of the House Armed Services Committee, disclosed at the hearing that the senior Pentagon position responsible for overseeing contracts with outside consultants working for Department of Defense and military suppliers has been vacant for 16 months.

Representative Aspin said this suggested a ``lack of interest'' in curbing abuses.

Other lawmakers are concerned about the costs and number of consultants. Senator Pryor, for instance, wants more competitive bidding in hiring consultants and better accounting of what they charge.

New regulations may come out of the current focus on consultants. One idea being backed by Pryor and others would force consultants to register, similar to the way lobbyists have to. Others are looking at ways to tighten the so-called revolving door regulations that govern those who go from the Pentagon to private business.

Consultants, however, bristle at the idea of more regulations. ``There are plenty of strictures there,'' says William Richardson, a retired four-star Army general who works for a defense consulting firm here. ``You just have a few people who broke them.''

What the Pentagon spends for consulting In millions of dollars for fiscal year 1988 Contracted, advisory and assistance services $ 11 Studies, analyses, and evaluation 254 Management support 970 Engineering and technical support 380 Information technology 584 Systems engineering contracts 943 Federally funded research and development centers 725