Two doses of buspirone (5 and 10 mg tds), 1 dose of diazepam (5 mg tds) and placebo were administered to 8 normal subjects for a period of 8 days. Each subject received each drug in a balanced order with a minimum interval of 1 week between courses. Psychotropic effects were assessed with a battery of physiological, psychomotor and subjective tests on the first, third and last days of treatment both before the first daily dose and at 1 h and 3 h after it. Diazepam showed a characteristic profile of action producing EEG changes and psychological impairment after a single dose which were still present after a week's treatment. Such effects were minimal after buspirone. Both drugs increased subjective ratings of drowsiness but these feelings tended to decrease after a week's treatment on the clinical doses. Buspirone (10 mg tds) produced some unpleasant side-effects initially but tolerance to these invariably developed after 3 days treatment.