Meperidine increases the sleeping time induced by pentobarbital in guinea pigs. The effect of pentobarbital on the toxicity of meperidine was antagonistic, whereas the effect of meperidine on the toxicity of pentobarbital was nearly additive. The results of these experiments show that pentobarbital will protect experimental animals against the convulsive effects of meperidine and that the depressant action of pentobarbital is augmented by the presence of meperidine It has been pointed out that when meperidine and procaine are combined there is a potentiation of convulsive effects, but their combined lethal effects, while greater than that of either one alone, are not additive.