Democratic Costa Rica has rebuffed a hint of military aid from the Reagan administration because, among other things, it has no army.

Costa Rican President Rodrigo Carazo, in an indignant letter to U.S. Ambassador Francis J. McNeil, described his country as "pacifist in word and deed" and determined to remain so. The letter, made public in San Jose, said Costa Rica is prohibited by its constitution from having an army, and that its people are convinced that "only through justice will they gain peace."

Carazo's ire was generated by press reports quoting U.S. Ambassador to the United Nations Jeane J. Kirkpatrick, on a recent trip to Latin America, as saying that the United States would be willing to consider a program of military aid to Costa Rica.

Kirkpatrick, in a news conference in Lima, Peru, volunteered that "the destabilization of Costa Rica demonstrates that democracies as well as non-democracies are vulnerable." According to a State Department transcript, she went on to say that the U.S. government "is prepared to help such nations defend themselves, and to do so by several means, namely economic aid, military assistance and longer range economic assistance programs such as the Caribbean Basin plan."

In recent years the United States has supplied defense articles to Costa Rica's small "public security force" through the foreign military sales program, and this year Congress has been asked for $30,000 to finance U.S. military training for Costa Rican air and sea rescue units.

In recent months Costa Rica, which is considered the most stable democracy in Latin America, has witnessed increased terrorism and political violence. Nevertheless, State Department officials said yesterday that Washington is "not proposing anything new" in the way of aid, and that a letter to Carazo is being drafted to say so.