The administration of trycyclic antidepressants followed by reserpine-like drugs elicits a pattern of stereotyped locomotor activity. Using desmethylimipramine (DMI) followed by tetrabenazine (TBZ), activity could be reliably elicited only in young rats, and only by using very high doses of TBZ. The latency of onset of activity was up to 5 h. Animals rendered active by DMI-TBZ failed to perform a well-established operant task. The activity syndrome was apparently unaffected by pre- or posttreatment with the dopamine antagonists spiroperidol or primozide, but partial antagonism was obtained with the noradrenaline antagonists phentolamine and phenoxybenzamine. The findings are discussed in relation to the mechanism of action of the effect and its relevance to the clinical action of tricyclic antidepressants.