The mother of a Soviet defector attempted suicide by swallowing acid yesterday after officials refused to allow her and her family to join the son in Sweden.

Antonina Agapova, 68, was reported in extremely serious condition in the poison ward of a city hospital. She apparently brought a bottle of acetic acid with her to a Soviet passport office and drank it after the official refusal.

Agapova, her daughter-in-law, Ludmilla, 39, and granddaughter, Libya, 13, have tried four times in the past four weeks to escape by light plane over the Finnish border from a small, frozen lake north of Leningrad. Each time, bad weather or bad luck thwarted them.

Ten days ago, two men were arrested by Finnish authorities at a small the Soviet Union on charges that they flew a light plane through restricted airspace in their return flight.

The three women have been trying to join Valentin Agapov, a Soviet merchant sailor defected to Sweden when his shop docked there in 1974. Agapov has been granted asylum. Ludmilla Agapova told the reporters last week that she had decided to risk illegal escape after the Soviets repeatedly had refused her family permission to emigrate.

Yesterday, the defectors mother and wife submitted new exit visa applications to the passport office, according to two Western correspondence who witnessed the suicide attempt.

Ebba Savbord, a Swedish journalist, said that when Antonia Agapova learned of the refusal, she left the passport office waiting room, went to a bathroom and emerged a few minutes later, holding an empty bottle marked acetic acid. She then collapsed on the floor.