Pinch-induced catalepsy and activity of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase, in brain structures have been studied in mice of six inbred strains. A pronounced predisposition to catalepsy was found in the CBA mouse strain. It was shown that the CBA mice had the highest tryptophan hydroxylase activity in the neostriatum compared to mice of other noncataleptic strains. The experience of repeated victories in intermale encounters producing highly aggressive CBA mice resulted in the inhibition of the genetically determined predisposition to pinch-induced catalepsy and in the simultaneous decrease of tryptophan hydroxylase activity in the neostriatum down to the level found in noncataleptic mice. The inhibitor of tryptophan hydroxylase, p-chloromethamphetamine, significantly decreased the enzyme activity in the neostriatum of CBA mice and completely inhibited their genetically determined predisposition to catalepsy. These findings indicate the key role of the striatal serotonergic system in the catalepsy-inducing mechanism.