At a bustling assembly plant here, Brazilian workers mount 90 MM cannons on the chasis of armored combat vehicles designed and constructed by Brazilian engineers. Destination: Libya.

A few miles away, at the sprawling production center and proving grounds of an aircraft company owned by the Brazilian government, technicians build a special order of Bandeirante maritime military reconaissance planes. Destination: Chile.

Business is booming at these arms plants, and that is good news for Brazil's military rulers. With every armored car or airplane that rolls off an assembly line, the government's campaign to free itself from dependence on foreign arms suppliers is a step closer to fruition.

While the main goal is apparently for Brazil to be able to supply its own armed forces, as a byproduct Brazil is also emerging as an explorer of military equipment.

"It's really just a matter of simple economics," explains a U.S. military official. "The domestic market here isn't big enough to support an arms industry, so to get the economy of scale they need in order to make their operation viable, the Brazilians have had to go overseas. The more they sell outside, the cheaper the cost for themselves."

According to U.S. military sources in Rio de Janeiro, this export program is still a "fledgling. It is growing rapidly, however, and Brazil already has foothold in the Latin American, Middle Eastern and African armaments markets.

The biggest single deal has been with Libya. In an agreement that became known early this year, Engesa, a privately-owned manufacturer of military and civilian vehicles, sold several hundred armored reconnaissance cars to the Libyan army at a price that a foreign military observer put at "well over $100 million."

That deal came shortly after the tiny Arab sheikdom of Qatar had ordered 20 of the same vehicles.

Although engesa has not disclosed the exact number of Cascavel cars sold to Libya, a U.S. military source says the deal calls for "an initial order of 200, with perhaps as many as 500 more to follow."

The first shipment of cars has already been incorporated into the Libyan armed forces, receiving their baptism of fire in July, during Libya's border clash with Egypt. According to a Brazilian army publication, the cars "proved themselves very well" - a judgment most foreign military observers are inclined to accept.

"The thing that speaks best about the performance of the Cascavel," says one U.S. military man, "is that since their debut this summer, one Islamic state after another has sent missions over here to talk about buying them. There wouldn't be that kind of interest if they hadn't done well."

Late last month, for example, delegations from Iraq and Turkey were reported to have met with the head of the Brazilian army's War Material Department and to have attended demonstrations here of the Cascavel and the Urutu, an Engesa-made amphibious personnel carrier, Brazilian press reports have also said that Pakistan and Indonesia have shown interest in buying Engesa armored vehicles.

"I think the Brazilians are going to be selling a lot more of the Cascavel and Urutu to Third World customers," says a U.S. military official.

"Aside from the fact that they're cheap, they're mobile and easy to maintain, just what the doctor ordered if you've got a limited defense budget or a small road network and a lot of rugged terrain."

The Third World has also been the best market for the state-controlled Brazilian Aeronautical Co. (Embraer). Particularly successful has been the versatile Bandeirante plane, Embraer officials stressed that it is able to operate independent of ground support from the short, rough, remote runways often found in Third World countries.

Embraer's first export sale, in 1975, was of five Bandeirantes to neighboring Uruguay. Since then, Chile has bought nine, costing $9 million, for its navy.

To increase the export potential of the Bandeirante, Embraer also manunfacturers a civilian version at its main plant here. This plane has been bought by commuter airlines in Sudan, Britain and France, and Embraer officials say negotiations are under way with potential buyers in the United States, Australia and Greece.

Embraer has also begun exporting the Xavante jet fighter used by the Brazilian air force since the early 1970s. It sold three Xavantes to Togo in a $5.7 million deal that also calls for Embraer to provide parts and service and to train Togolese pilots here.

"Africa now looks to be our most promising market," says Embraer export director Heilor Fernades Serra. "We have met some resistance here on our continent because the Andean pact countries are planning to build planes in Columbia and thus prefer not to buy from us."

The other Andean Pact nations are Venezuela, Ecuador Bolivia, Paraguay, Uruquay and Chile, all of which, like Brazil, are ruled by rightist military, governments show no sign of diminishing, however.

"All of our military export sales require the prior approval of the Ministry of Foreign Relations," says Serra. "But aside from certain zones in which there are countries disputing frontiers, such as Central America, we have been encouraged by the government to sell whatever we can, wherever we can."

Embraer and other arms exporters say foreign military observers> are likely to gain even more official support in the near future. Brazil is driving to improve its balance-of-payments situation and Brazilian officials are said to see exports of military hardware, especially to the Middle East, as one of the best ways to get back some of the money spent on the importation of petroleum.

"Brazil is years away from being an arms exporter in a class with England, France or West Germany," says an American military official. "But based on what we've seen there's nothing they'd like better."