The hypnotic properties of leaves and seed of Casimiroa edulis (zapote blanco) a tropical fruit, are well known. In the present investigation, pharmacological studies on aqueous and alcoholic extracts were carried out in dogs, cats, rabbits and guinea pigs, using different experimental models. The action of the extracts upon blood pressure, cardiac activity, respiration rate, muscular tone, electroencephalographic activity, sleep and other parameters were studied too. In addition to, studies upon isolated human, cat, rabbit and guinea pig tissues gave information about the extracts effect on smooth muscle activity. It was found that both aqueous and alcoholic extract of the seed of zapote blanco produced marked hypotension in all animal species, which was followed by a sleep-like stage. Also a definitive oxitocic effect in vivo and in vitro was demonstrated suggesting the presence of a potent uterus constricting substance.