CRL 40028 (benzhydryl sulfinyl acetohydroxamic acid)-induced stimulation in mice, measured as an increase in locomotor activity was studied following an intraperitoneal injection of drugs known to block alpha-adrenergic receptors. Phenoxybenzamine (20 mg/kg), prazosin (0.5-1 mg/kg) and yohimbine (2 mg/kg) prevented the development of CRL 40028-induced hyperactivity. These results suggest that stimulation of an alpha-adrenergic postsynaptic receptor is of importance for the stimulant effect of CRL 40028.