The ability of the substance P (NK1) receptor antagonist CP-99,994 to alter catalepsy induced by a dopamine D1 or D2 receptor antagonist and the ability of CP-99,994 to influence acetylcholine (ACh) release in the striatum were investigated in rats. Catalepsy produced by the D1 antagonist SCH 23390 (0.5 mg/kg s.c.) was not altered by CP-99,994 (0.5, 2.5, or 10 mg/kg s.c.). In contrast, catalepsy induced by the D2 antagonist raclopride (2.5 mg/kg i.p.) was attenuated by CP-99,994 (2.5 and 10 mg/kg). CP-99,994 (10 mg/kg) did not stimulate locomotion when given alone. The less active enantiomer of CP-99,994, CP-100,263 (10 mg/kg), did not alter raclopride-induced catalepsy. Both systemic administration and intrastriatal perfusion of CP-99,994 alone decreased striatal ACh release. Bilateral intrastriatal perfusion of CP-99,994 (40 and 100 microM) reduced catalepsy produced by raclopride and attenuated raclopride-induced increases in striatal ACh release. The reductions in the duration of catalepsy and decreases in striatal ACh release associated with CP-99,994 perfusion were positively correlated. These findings suggest that blockade of striatal NK1 receptors reduces catalepsy induced by a dopamine D2 antagonist, an effect mediated, at least in part, by reducing striatal ACh release. Striatal NK1 receptors, therefore, may be a new therapeutic target for developing drugs that alleviate motor side effects associated with antipsychotic treatment.