Corticosterone slow-release pellets, implanted for 9 weeks in young Fischer 344 rats, resulted in continuous high plasma levels of the hormone which are comparable to those of rats under mild stress. One week following termination of the drug treatment, the rats were tested in an eight-arm radial maze. During the initial acquisition stages, corticosterone-treated rats exhibited cognitive impairments in contrast to placebo-treated rats. The deficits were observed in all three parameters which were monitored, the total number of errors, the number of correct entries out of the first eight, and the total time needed to complete the test. This study is the first to report specific behavioral decrements related to the previously observed morphological hippocampal changes induced by long-term corticosterone administration.