When Bob Cox returned home last Nov. 19, he found his 10-year-old son Peter crying and the rest of the family in hysterics. "We've got to get out, dad," Peter said through his tears.

Robert Cox, 46, editor of the English-language Buenos Aires Herald for over a decade, winner of Columbia University's prestigious Maria Moors Cabot award for distinguished reporting from Latin America, recipient of the Order of the British Empire from Queen Elizabeth Ii, is no stranger to death threats.

Usually they have come anonymously by phone or mail to his office with the same depressing message: "Get out or be killed, you Jew-Communist son of a bitch."

But this time, the letter, purportedly from the left-wing Montonero guerrillas, was addressed to his young son and Cox could not shrug it off. It contained far too much information that Cox believes could only have been gathered, not by the lefist urban guerillas, but by Argentina's military intelligence services.

Last Sunday, Cox joined a long and distinguished list of Argentine journalists silenced by successive governments. The process began with Juan Peron's first regime in 1946 and intensified under the current military government, which came to power in 1976.

While there is hardly a country in Latin America where the press is absolutely free, Argentina now ranks with Uruguay and Haiti among Western Hemisphere nations where objective journalism can be a dangerous business.

Cox, his wife, Maud, and their five children were forced to leave Argentina for London, where they began at least a year in exile. Their departure followed 24 months of harassment by men in civilian clothes who attempted to gain entrance to their apartment, accosted a relative's maid for information and frightened the family so badly that they spent several nights hiding with friends or at embassies here.

The Cox case has once again focused attention on the limits placed on the press in this country and on the more than 100 journalists who have been arrested, forced into exile or who have disappeared since 1976.

Contrary to the government's assertions, freedom of the press is strictly circumscribed in Argentina today. A murky form of "self-censorship" -- the limits of which are set for the most part by phone calls and unsigned memos from the military authorities, rather than by codified laws -- keeps the press in line.

Journalists and editors who repeatedly fail to heed the instructions they receive, who print more than they should when there are no specific instructions, or who simply "know too much," may suddenly disappear or find themselves, as in Cox's case, so threatened that they choose to leave.

There are usually warnings before such drastic measures are taken. Cox was arrested two years ago, stripped and subjected to various indignities for 12 hours before he was released. The publisher of La Prensa, the most independent Spanish-language newspaper here, has been tried and acquitted several times for violating a law that prohibits any mention and the Montonero except when their names occur in official communiques.

Last week, Gen. Osvaldo Cacciatore, the military mayor of Buenos Aires, announced he would start legal proceedings against Siete Dias magazine because, he said, it had portrayed "a negative image [of him] which injures his post and his standing in the community's opinion."

This week, all of Siete Dias' press run was seized by the Morality Division of the municipality of Buenos Aires, without explanation. The action went unreported in all of the city's newspapers except for La Prensa and The Herald, which continues its campaign for human rights and freedom of the press despite Cox's departure.

Argentine journalists say that, despite an improving climate and a greater tolerance over the past year for what may be reported and criticized editorially, they still work in an atmosphere of fear. "This is not for quotation," said one reporter who agreed to talk anonymously. "I'd be killed."

Only Hector Grossi, executive editor of Conviccion, a newspaper with close ties to the Argentine Navy, was willing to discuss for the record the conditions under which the press here works.

"I admit that there is a high level of self-censorship." Grossi said, although he added that he and other editors have recently been testing the limits and ignoring official instructions, without reprisal so far in his case.

Nonetheless, the oral and written instructions still come from the office of the General Secretariat for Public Information, which is headed by a general and is located in President Jorge Videla's office.

Newspapers here, for example, were told earlier this year not to portray the Sandinista guerrillas fighting against the Somoza regine in Nicaragua as "heroes." The press took the hint and pictured them as Cuban-supported communists determined to overthrow a beleagured, anticommunist government abandoned by the Carter administration.

At least 70 "disappeared persons" are journalists. A number of them are former staff members of La Opinion, a newspaper run by Jacobo Timerman, who was himself arrested, tortured and kept confined for over two years before he was expelled from Argentina in September. Timerman, like many of his colleagues, was "guilty" of "intellectural subversion" in the eyes of the military. In several instances over the past few years, the bodies of "disappeared" journalists have been found riddled with bullets, alongside little-used roads.