Cimetidine, an H2 receptor antagonist, administered into a lateral ventricle of the rat brain caused a significant increase in tail flick latency. Dimaprit, a specific H2 agonist, failed to counteract the analgesic effect of cimetidine. In contrast, it enhanced the effect of cimetidine and per se had marked analgesic activity. The specific opioid antagonist naloxone was without effect. Pretreatment with CaCI2 completely prevented the action of cimetidine. These findings suggest that the analgesic action of cimetidine and dimaprit is not due to specific effects on H2 receptors.