Despite the recent release of a number of political prisoners, church groups and others are pressing the Chilean government for information on the whereabouts of men and women who have disappeared and who are believed to have been taken into custody by the secret police.

Fifteen prominent writers and other public figures have sent an open letter to Supreme Court President Jose Maria Eyzaguire Jorge Alessandri expressing concern about history professor Fernando Ortiz, one of eight persons who disappeared Dec. 15.

A government spokesman said yesterday that Ortiz, 54, a former official of the University of Chile professors association, is not in custody.

The disappearance of the eight persons, believed to be leaders of the underground Communist Party, was the first such incident since more than 300 political prisoners were freed in November and the government announced the "total liberation" of all persons held without charges.

Human rights groups here say that a total of 13 persons have disappeared since the polictical prison camps were emptied Nov. 18. Habeas corpus petitions filed by relatives with the assistance of a Catholic church legal agency alleged that the missing persons had been arrested by the National Intelligence Directorate, known as DINA, the initials of its name in Spanish.

Meanwhile, the church filed writs in the Supreme Court on Christmas Eve requesting a special investigation of the cases of 415 missing persons allegedly arrested by DINA. It was the tenth such request by the church's Vicarate of Solidarity in the past 15.

The court had refused to comply with the vicarate's request in October for an investigation of 383 disappearances. At that time the church lawyers gave the court a 700-page study of the problem of persons who have vanished in Chile since the military overthrew the leftist government of Salvador Allende in 1973.

An average of 10 Chileans disappeared every month during 1976, according to church sources. Habeas corpus petitions are routinely rejected by the court when the Interior Ministry says that the person being sought is not in custody.

The open letter to former President Alessandri and Supreme Court President Eyzaguirre was signed by Matilde Neruda, widow of Nobel Prize-winning poet Pablo Neruda; former Education Minister Maximo Pacheco; former ambassador of the Organization of American States Alejandro Magent and others.

The letter said that relatives of Ortiz "has grounds to presume he was detained, since members of the 'security services' had asked for him directly or indirectly on various occasions in different places and homes of relatives."

Also among the eight persons who disappeared Dec. 15 is Edras Pinto, 48, a former secretary of Communist Party leader Luis Corvalan, who was released Dec. 17 in exchange for the freedom of Soviet dissident Vladimir Bukovsky.

A similar roundup of Communist leaders occurred May 12, a few days after the visit to Chile of U.S. Treasury Secretary William Simon. Their whereabouts are still not known.