1. The effect of the nootropic agent adafenoxate (a structural analogue of meclofenoxate) on the binding parameters of 5-HT1 receptors in vitro and in vivo in rat cerebral cortex, striatum, hippocampus and hypothalamus was studied. 2. The chronic (100 mg/kg per os for 7 days) adafenoxate treatment produced a significant (24.6%) decrease in the density of 5-HT1 sites in the hippocampus. 3. In vitro adafenoxate inhibited specific [3H]5-HT binding with equal potency in all the regions studied with IC50s in the microM range. 4. It is suggested that the decrease in the density of the 5-HT1 sites in rat hippocampus might contribute to the nootropic action of adafenoxate.