In rats and mice the serotonin and histamine antagonistic drug 9,10-dihydro-10-(1-methyl-4-piperidylidene)-9-anthrol (WA 335-BS) caused stronger central sedative effects than did cyproheptadine. WA 335-BS also displayed stronger activity against reserpine- and central tremorine-induced effects than did cyproheptadine and it slightly enhanced d-amphetamine-induced effects: therefore it may have antidepressant properties. WA 335-BS proved to be very effective against isolation-induced aggression in male mice. The comparatively small anxiolytic effects may have been caused in part by the central antiserotonin properties. Like cyproheptadine, WA 335-BS increased food consumption in cats. In EEG-experiments in the conscious rabbit the serotonin-antagonistic drugs WA 335-BS and cyproheptadine exerted stronger depressant activity on the arousal reactions than did the neuroleptic chlorpromazine. The results of our animal studies suggest WA 335-BS to be an antidepressant with sedative properties.