Mexican and US presidents: a great deal to talk about

As they stroll in the presidential mountain retreat at Camp David today, Ronald Reagan and Jose Lopez Portillo of Mexico test and ability of personal diplomacy to solve longstanding problems.

* Trade: After Japan and Canada, Mexico is America's third-largest trading partner, accounting for to $27.5 billion a year. The US wants more of Mexico's oil, a resource that country intends to husband for its own long-range development interests. But Mexico needs easier access to US markets to meet a $ 2.5 billion export-import deficit, and its internal stability depends on smooth trade relations.

* Central America: Mr Lopez Portillo, in the final 18 months of a six-year term, has repeatedly opposed outside intervention, by the US as well as others, in Central America. Left-wing groups have been fighting in El Salvador, Guatemala, and Honduras with supplies alleged to come from Cuba and the Soviet Union. Mexican foreign policy is traditionally founded on suspicion of its big northern neighbor, and Mexico has retained friendly diplomatic contacts with Havana. It is a sensitive situation for both Mexico and the US.

* Immigration: Mexico, which shares a 2,000-mile border with the US, is bursting at the seams, with a population of about 75 million, which is growing at an annual rate of around 3.4 percent. The US, with 220 million people, has a growth rate of around 0.8 percent (before immigration). A chaotic condition exists in the US Border Patrol. which is short of funds and unable to halt illegal immigration, running from half a million to a million or more a year.

Reagan is offering Mexico an increase in immigration quotas from the present ceiling of 20,000 to 50,000 a year, and a two-year test of a guest-worker program, with temporary admission of a million workers. Mexican immigration, legal or illegal, is regarded as a "safety valve" for internal stability in a country of marked social disparity and extensive unemployment.

The new immigration proposals face difficulties: Mexican population growth, though declining, is still so big that the new proposal may be inadequate; the plan requires worker- registration and a tightening of the border patrol; Congress must approve over the opposition of US organized labor. US unemployment is rising it is noted and illegal Mexican immigrants in the US are variously estimated as from three to six million.

* Third world: An international group, under the chairmanship of former West German Chancellor Willy Brandt, declared in a 1980 report that North-South economic disparity has produced "much greater dangers than at any time since the Second World War." The situation has deteriorated since then, with enlargement of third world external debts. Lopez Portillo bears an invitation to Mr. Reagan to attend a 23-nation North-South conference in October at the Mexican resort island of Cancun. Reagan will go, according to State Department advices. In the light of present US-Cuban relations, the Mexican government will not invite Prime Minister Fidel Castro to the meeting while President Reagan is there.

The Lopez Portillo visit is a test of whether Mr. Reagan can smooth a number of difficult international problems by personal diplomacy. President Carter used the "Camp David treatment" successfully in the summit conference on the Middle East.

Reagan and Lopez Portillo met in Mexico City in 1979 and again this Jan. 5 in Ciudad Juarez. After the present two-day conference here they will see each other again under current plans, at the North-South conference this autumn.

President Reagan wants another, tripartite parley, which would include a third member, Canadian Prime Minister Pierre Trudeau. No place or date has been set for this proposed North American summit.