Intraperitoneally administered morphine induced catalepsy in mice. Morphine pretreatment however, failed to antagonize apomorphine-induced cage climbing behaviour thereby ruling out the possibility of its possessing DA receptor blocking activity. Pretreatment with L-histidine, a precursor of histamine, and atropine, potentiated the cataleptic effect of morphine whilst pretreatment with chlorcyclizine, an H1 receptor blocker, and naloxone, a morphine antagonist, antagonized morphine-catalepsy. Pretreatment with metiamide, an H2 receptor blocker, and methysergide, a 5-HT antagonist, did not significantly alter the cataleptic effect of morphine. The results with L-histidine and chlorcyclizine suggest an involvement of central histaminergic mechanisms in the cataleptogenic effect of morphine in mice. Further, as the cataleptic effect of morphine was also antagonized by naloxone it appears that the interaction of morphine with the central histaminergic mechanisms is mediated through specific opiate receptors.