In 1943, the crossroads in front of the Patuxent Naval Air Test Center was not much more than a string of bars and a few chowder houses.

But in four decades, Patuxent has became one of the East Coast's major aircraft and weapons testing sites, and the industry serving it has mushroomed.

Today, the facilities of 100 defense contractors and consultants dot the terrain here. Their employes work on weapons, communications, and sensor and radar for naval aircraft, and they test the systems before the planes are mass-produced.

Bendix, TraCor, Martin-Marietta, Vitro, RCA and DynaMac are among the high-tech communications and engineering fields that have built offices in Southern Maryland. Since 1980, the Reagan administration's defense buildup has pushed military and civilian employment here up 15 percent.

At a conservative estimate, 12,000 of St. Mary's 64,500 residents work for the Navy, the technical-industrial base or for housing, recreation and food services with ties to the Navy.

There are 3,700 military families in this area, only 709 of whom live on the base.

So when local officials estimated that 500 jobs are "immediately at risk" unless a $1.3 billion congressional cap on Department of Defense consulting contracts is lifted by March 31, "it caused a panic in the business community," said George Havens, executive director of the local Chamber of Commerce.

Last week, Rep. Roy Dyson (D-Md.) sponsored a bill to lift that cap, but Southern Maryland officials are not optimistic about its passage. They are uneasy that current contracts will not be honored, leaving the local economy in disarray.

"The Navy is the locomotive that pulls our local economy," said David Morgan, director of economic development for St. Mary's County. "The threatened loss of 500 jobs means a lot in a community this size."

In recent years, St. Mary's officials say, demand for housing, goods and services, schools and recreation have grown as workers have poured into the area.

Between 1982 and 1984 the number of military personnel assigned to the base grew by 400 to the present 3,792; civilian employment went up by 200 to 3,786, and the number of civilian contractors working on defense support projects rose about 200 to 3,000, base public information officer William Frierson said.

County administrator Edward Cox said that if the current defense buildup grinds to a halt, the ripple effects locally "would be endless . . . . We're talking about the potential loss of $15 million in salaries."

"About 4,000 new housing units are somewhere in the pipeline, and a substantial portion would be used by those on the Navy base," Morgan said, noting just one sector that would be adversely affected if budget limits were to spike a full-scale pullout of local defense contractors and consultants.

"Besides the obvious economic impact, these people represent a pool of local talent and skills, ideas, energy and money that we don't want to lose," Cox said.

"Small companies like Dalfi -- of which St. Mary's has 50 or 60 -- are potentially hardest hit," Morgan said.

Corporate managers at Dalfi's Philadelphia headquarters sent 60 of the 126 employes here home Jan. 15 after the Navy told them money for the company's 30-month, $10 million contract was in doubt, according to St. Mary's operations head Edward Welch, who lost 12 workers at his field site.

Although 22 employes are now back at work in offices in Virginia, California and Italy, Welch said, "It's almost impossible to pick up where you left off. These people left and got other jobs.

"The work's not getting done or is backed up. Everything won't be all right even if the Dyson bill goes through. People are now very wary of working in a contractor situation."

TraCor's chief in Southern Maryland also maintains that the region stands to lose if confidence is not swiftly returned to the national defense contract scene.

"Everyone drives through here and sees the beautiful farmlands, the rural vistas and seascapes, and thinks 'watermen and tobacco.' But really, this is a boom area in high-tech for the entire state," said TraCor operations manager Jesse Hawes.

Hawes said that TraCor, which has 575 workers and is one of Southern Maryland's largest employers, has in mind a "worst-case scenario in which 175 jobs would be in jeopardy" but has no immediate plans to lay off anyone.

Hawes said that since word of the freeze first came from the Navy he has had "at least three contracts recoded" so they no longer fall into the category specifically targeted by the freeze. Other contracts have been granted money for 30 days to keep them afloat.

"It's a cash-flow problem that many small companies share," he said. "We staffed up for a specific project. Now the money has stopped, so the question becomes, how long can you afford to hold onto your people?"