In the slang of Brazilian politics, military men who have retired and developed successful second careers in business and government are known as "amphibians." As comfortable in the civilian world as in the military, the shuttle between key public positions and private enterprise.

Exactly how many there are, no one seems to know, and efforts to find out prevails here. The Brazilian government asserts that the armed forces are merely "the people in uniform" and denies that any segment of the military has been transformed into an elite caste by its years in power.

Observers here agree, however, that the number - and influence - of the "amphibians" is greater today than at any time since the military seized power in 1964. By offering themselves as a link between their former military colleagues and the civilian politicians and businessmen whose support the government courts, the "amphibians" have made themselves indispensable.

"They have clearly become the power behind the throne in Brazil," says a diplomat in Brasilia. "They like to keep a low profile and stay out of the spotlight but wherever real power has accumulated or money is being made or spent, you'll find at least one of them at the center of things."

As brokers of power and influence, the amphibians have no equals," adds Rio banker. As businessmen, bureaucrats or politicians, they are technically civilians, but let's face it: a military man doesn't suddenly become unplugged from the system just because he retires."

The best known and most strategically placed "amphibian" is Golbery do Couto e Silva, 66, a former army general whose post-retirement years have been spent in the upper ranks of business and the government bureaucracy. He is currently serving as chief of President Ernesto Geisel's "civilian household" - a post that gives him and foreign policy.

Considered the intellectual architect and chief theoretician of "the revolution," as the government here likes to style itself, Couto e Silva headed a business "think tank" after leaving the army and later founded the National Intelligence Service, with functions similar to those of both the CIA and FBI.

In the mid 1960s, he became president of the Dow Chemical Co. of Brazil, a job helf until being invited back into government in 1974 by his old friend, Gen. Geisel.

A Sao Paulo lawyer who had dealt with Couto e Silva on human rights issues calls him "the second most powerful man in this country. Nobody has as much say-so in as many policy areas, and besides, he's probably the only man in Brazil who can get away with calling Geisel 'Ernesto.'"

Following Couto e Silva's example, other "amphibians" ensconced in comfortable posts in business have come into government to serve on Geisel's staff. Maj. Heitor de Aquino Ferreira, now Geisel's private secretary, was employed for several years after leaving the army as an executive at the Jari Project, American billionaire Daniel Ludwig's controversial multi-million dollar Amazon development program.

Observers point out that the same pattern exists in other key sectors of the bureaucracy, such as the presidential cabinet. Communications Minister Euclides Quandit de Oliveira, a former naval captain, come to his post from the board of directors of Siemens, S.A., the Brazilian subsidiary of the giant West German electrical and electronic equipment manufacturer.

A few military retirees have also linked up with the ruling government party and have won elective office. Retired Gen. Ney Braga, for instance, has sandwiched terms as a federal senator and state governor around stints as chief of the agriculture and education ministries. He is frequently mentioned as a possible presidential choice should the military have decide to hand power back to civilians.

But the favorite nesting place of the "amphibians" continues to be the corporate community. For both Brazilian firms and the multinationals that have been drawn by the "economic miracle" that has given Brazil the tenth largest economy in the world, retired military officers are seen as a convenient way to gain access to the power structure.

"It is a recognized fact that since 1964 military men have come to occupy posts of prominence in companies installed in Brazil," said the Rio newspaper O Globo in a recent report on the growing number of retired military men in private enterprise. "At present, it can be ascertained that the main interest of the businessman lies in the case with which the military group can establish contacts."

Some especially well-connected "amphibians" have been able to go into business for themselves. Col. Mario Andrezza, former minister of labor and of transportation, is now a partner in a construction company and also serve unofficially as a political adviser to Gen. Joao Baptista Figueiredo, handpicked by Geisel earlier this year as his successor.

Others have slipped into key posts at the dozens of public companies established to give added push to Brazil's export and industrialization drive.