The number of serotonin type 2 receptors (S2) was measured in the frontal cortex of mice belonging to 7 inbred strains using specific 3H-spiperone binding. In the same mice, measurements were also taken of the number of 5-hydroxytryptophan-induced (200 mg/kg i.p.) of head twitches (HT). A significant positive interspecific correlation was demonstrated between the number of S2 and HT. The conclusion is drawn that in the frontal cortex 3H-spiperone is bound to functionally active S2 and that the intensity of HT is largely controlled by the genetically determined number of S2 in the brain.