A Maryland court rejected an appeal yesterday by convicted rapist Richard Dale Baumgardner, 26, who was sentenced to life in prison in November for the kidnaping and assault of a 30-year-old Silver Spring woman.

Baumgardner, of Clarksville in Howard County, abducted and raped the social worker in January 1986, handcuffed and gagged her and tossed her off a bridge into the near-freezing Patuxent River. The woman managed to swim to shore, free one hand, climb a rocky embankment and hail a passing van driver.

Baumgardner was convicted in October on one count of attempted first-degree murder, two counts of first-degree rape, a first-degree sexual offense, two kidnaping offenses and several robbery and weapons charges. His sentence totaled four life terms plus 110 years.

The Maryland Court of Special Appeals rejected claims by Baumgardner's lawyers that the trial judge should have heard testimony about a "mental disability" that made Baumgardner believe that his victim consented to the sexual acts. Any mental infirmity less than legal insanity cannot be admitted as a defense in Maryland courts, the panel said.

The court also turned down arguments that the trial judge had given incorrect instructions to the jury that convicted Baumgardner and that the judge had improperly sentenced Baumgardner to consecutive life sentences for rape and a sexual assault.