Two years ago, the presence of IBM-PCs at the national computer conference here indicated a deliberate challenge to Brazil's closed market policy for high technology. This year, though, locally made machines are smoothly hooked up to IBM mainframe computers and the corporation's harmonious new message is "partners in progress."

For the 450,000 visitors to last week's fair -- now the world's third largest -- falling prices and the technical strides made by the local industry are a clear sign of the success of Brazil's controversial 1984 law closing the $845 million small computer market to outsiders for eight years.

"The big change is that no one here criticizes the law any more -- people are ashamed to do so," said conference President Paulo Roberto Feldmann.

For computer executives, the U.S. administration's current challenge to Brazil's high-tech trade barrier is proof that the information industry, now growing at 30 percent a year, has come of age. They smilingly maintain that President Reagan's criticisms of the Brazilian restrictions were "for domestic consumption only."

Though it has passed from open confrontation to more subtle business tactics, the struggle for control of the world's seventh-largest data processing market continues unabated. The "reserved market" policy's weakness is that the lack of investment in research creates a need for constant use of the innovations of U.S. technology to meet industry's demand for new products -- and vulnerability to control under the guise of cooperation and friendly technology transfer.

IBM, which dominates Brazil's nonrestricted, $880 million computer market, now proposes helping local manufacturers find markets for their products, hints at new technology transfers and is interested in buying locally made components.

A number of voices at the conference warned such tactics would encourage profit-oriented companies to corrode the trade barrier from inside, and that the policy was indeed in grave danger.

Once cooperation with multinationals increases and IBM takes over international marketing of Brazilian-made computers, "our industry will be in their hands," warned Edson Dytz, formerly head of SEI, the government's regulatory body for computer policy.

According to Fernando da Costa Azevedo, president of Brazil's state-owned computer corporation, Cobra, "If contacts with multinational companies reach a significant level, then they'd have to be condemned."

Azevedo said, "Reagan's speech will speed up position-taking in the Brazilian congress about a law governing the use of software. What we need is regulatory systems and some incentives so that we will be able to reply to society that the reserved market has been worthwhile."

Dytz said international pressure for Brazil to respect copyright laws and proposals in the General Agreement on Tariffs and Trade for rules governing use of software programs as part of wider concessions on international services also would threaten Brazil's independence.

Gray areas in the 1984 law will be redefined in a national computer policy document now being sent to the Brazilian congress that could result in a complementary software law. Currently, foreign interests control 90 percent of the $1.3 billion software market, but widespread piracy is weakening this grasp.

A crucial ambiguity in the law concerns telecommunications equipment and digital telephone exchanges. Communications Minister Antonio Carlos Magalhaes has lobbied strongly to reduce the power of the regulatory body SEI and to liberalize imports of new technology. SEI maintains that such equipment comes within the law and multinational companies importing it must furnish technical details.

Critics of Magalhaes fear that any success in getting telecommunications equipment out from under the law could act as a wedge to divide business interests and eventually destroy the reserved market.

Though last year's equipment sales totaled $1.7 billion, Brazil's overall spending on data processing, including software and personnel costs was $4.5 billion, according to conference officials.

Government agencies are responsible for one-third of this spending but are rapidly being overtaken by domestic banks, which are seizing control of the industry.

Under the former military government, computer policy was fostered by a curious alliance of military strategists, leftist members of congress and young Silicon Valley-style executives who marketed microcomputers copied from U.S. models while they preached technological independence.

Today, control of the protected industry is rapidly passing into the hands of banks modernizing their activities, with a future guaranteed by impeccable political connections.

Brazil's leading banks have invested in their own on-line systems, setting up subsidiaries that depend heavily on U.S. technology, and which also borrow from IBM's management and marketing style.

Itautec, the computer subsidiary of Itau bank, which is controlled by the family of Foreign Minister Olavo Setubal, is doubling in size each year.

In the fast-expanding market for on-line bank systems, Itautec's larger rival is SID, a company that has emulated IBM to the point of headhunting among its executives and adapting IBM's Charlie Chaplin motif to local circumstances. SID now controls more than half the market for terminals and has clear ambitions to launch itself internationally as Brazil's first computer major.

Like Itautec, SID is part-owned by a leading bank, and its political foothold is strong. It is controlled by Matias Machline, a member of President Sarney's "kitchen cabinet" of friends and informal advisers from the business community.

A rare critic of the law is Caio Mario Britto, director of Logus Computers. He maintains the law has simply sheltered a domestic cartel of businessmen like Machline whose eagerness to buy already proven operating systems and components is shortsighted.