The increase in density of 5-HT1c receptors in the choroid plexus of the third and lateral ventricles was analyzed in rats between the 1st and 90th postnatal days. An increase was found between the 3rd day and the 4th to 5th week, at which time adult values are attained. The development of the 5-HT1c receptors clearly differs from that of 5-HT1 receptors in other brain regions. The density in the choroid plexus is about 20% higher in the third ventricle than in the lateral ventricles.