Administration of serotonin (5-hydroxytryptamine, 5-HT) to pyramidal cells of the CA1 region of the hippocampus results in a hyperpolarizing response which is followed by a rebound depolarization and a decrease in the calcium-activated afterhyperpolarization (AHP). While the hyperpolarizing response has been previously shown to be mediated by receptors of the 5-HT1A subtype, the identity of the receptor(s) involved in the depolarizing response and decrease of the AHP have not been identified. In the present study the effectiveness of a series of 5-HT receptor antagonists in blocking the membrane depolarization and reduction of the AHP was assessed. While a variety of 5-HT1 and 5-HT2 antagonists were found to be ineffective, the substituted benzamide BRL 24924 was found to be a potent and selective antagonist of the 5-HT-induced depolarization and decrease in the AHP in this region. This effect however appeared unrelated to the ability of this compound to block 5-HT3 receptors, as ICS 205-930 and MDL 72222 were markedly less efficacious in blocking these effects of 5-HT. Upon blockade of 5-HT1A receptors, 5-HT elicits a depolarization which is accompanied by a marked increase in excitability. These effects were also dose-dependently antagonized by BRL 24924. The present results thus suggest the presence in the CA1 region of the hippocampus of a novel 5-HT receptor at which BRL 24924 functions as a selective antagonist and which is capable of mediating slow excitatory responses in central neurons.