The immediate posttraining intracerebroventricular injection of histamine (1 or 10 ng/rat) facilitated memory both of a stepdown inhibitory avoidance task, and of the habituation of rearing responses to an open field. As previously shown for the avoidance task, the combination of cimetidine (1,000 ng/rat) plus prometazine (1,000 ng/rat), but not each drug on its own, blocked the effect of histamine in the habituation task. The effect of histamine was not shared by the intracerebroventricular administration of the mast cell histamine releaser, 48/80 (0.1 to 100 micrograms/rat). The present findings indicate that the memory facilitatory action of histamine might be general across tasks, and that 48/80-releasable, presumably mast cell, endogenous histamine is probably not involved in memory regulation.