Various histamine H2 receptor antagonists were tested for their possible effects on intestinal motility of the dog both "in vivo" and "in vitro". They were found to behave quite differently from one another: ranitidine exerted a small atropine-sensitive stimulatory effect by itself and strongly potentiated the stimulatory action of compounds which interfere with the cholinergic system thus confirming its cholinomimetic direct and/or indirect activities. Cimetidine and mifentidine were ineffective even at very high doses. Oxmetidine exerted an inhibitory effect independently of the kind of the stimulant agent and this suggested a direct effect on the smooth muscle. The different effects elicited by the above H2 antagonists indicate that their action on the intestinal motility is completely independent of the H2 blockade. This was furtherly confirmed by the lack of effect shown by the H2 receptor selective agonists, dimaprit and impromidine. The motor effects of ranitidine were, so far, the only ones which have been encountered, in different conditions, also in humans.