A depressed state can be induced in mice by forcing them to swim in a narrow cylinder from which they cannot escape. After a brief period of vigorous activity the mice adopt a characteristic immobile posture which is readily identifiable. Immobility was reduced by tricyclic antidepressants, monoamine oxidase inhibitors and atypical antidepressants, as well as by electroconvulsive shock. Psychostimulants also reduced immobility but in contrast to antidepressants caused marked motor stimulation. Immobility was not affected by minor or major tranquilisers. These findings, closely parallel to those we have previously reported in rats, suggest that the procedure is selectively sensitive to antidepressant treatments. The mouse procedure is, however, more rapid and less costly than that with rats and is thus more suitable for the primary screening of antidepressant drugs.