The stimulation of central serotonin receptors leading to a decrease in blood pressure is well documented, but the nature of these receptors is not well known. It was therefore decided to characterize such receptors by assessing the modifications in blood pressure and heart rate after the intracerebroventricular administration of several drugs to anesthetized cats. Serotonin produced a dose-related decrease in blood pressure and heart rate, while para-chloroamphetamine produced a slowly developing increase in blood pressure, and quipazine did not affect any of the parameters measured. The hypotension produced by serotonin was not modified by pretreatment with methysergide, was partially antagonized by quipazine, and was completely blocked by tolazoline. The present findings suggest that the central serotonin receptors which modulate blood pressure are different not only from peripheral receptors, but also from other receptors of this type.