Prenatal 5HT depletion causes a significant decrease in the level of nociceptive sensitivity during the second phase of the formalin test behavioral response. These experiments were designed to test whether blocking 5HT 2A/2c receptors in the CA1 region of the hippocampus and dentate gyrus would decrease nociceptive behaviors induced by a peripheral noxious stimulus formalin as an animal model of unremitting human being. The 5HT 2A/2c receptor antagonist ritanserin (2, 4 and 8μg/0.5μl) was injected into the CA1 area and dentate gyrus of behaving rats 5min before subcutaneous injection of formalin irritant. Nociceptive behaviors in both phases of the formalin test were significantly decreased by ritanserin (4 and 8μg/0.5μl) and ritanserin had no effect at 2μg/0.5μl. These results support the hypothesis that the hippocampal formation may modify the processing of incoming nociceptive information and that 5HT 2A/2c receptor-sensitive mechanisms in the hippocampus may play a role in nociception and/or the expression of related behaviors.