Forcing a rat to swim in a situation from which there is no escape results in an increase in plasma corticosteroid level. This rise was selectively inhibited by benzodiazepines, phenobarbital and meprobamate but not by other psychotropic drugs like trycyclic antidepressants, monoamine oxidase inhibitors, neuroleptics and amphetamines. This effect of benzodiazepines is of central origin since diazepam did not block the rise in plasma corticosteroid level produced by adrenocorticotrophic hormone. Diazepam also had no effect on plasma corticosteroid levels in hypophysectomized rats or in rats treated with betamethasone. Brief stress did not alter binding of [3H]diazepam in vitro nor binding of [3H]flunitrazepam in vivo.