The effect of endogenous hyperprolactinemia induced by pituitary transplantation under the kidney capsule on haloperidol induced catalepsy was evaluated in male Wistar rats treated with two doses of the drug (500 gamma/kg; 2 mg/kg i.p.). Rats of 220 +/- 30 g received intraperitoneal injection of haloperidol. Every five minutes following drug administration the rats were assessed for catalepsy by placing the forepaw on a horizontal bar, and observed for two minutes. Data obtained show that hyperprolactinemia potentiates the cataleptic score in rats treated with dose of 500 gamma/kg i.p., while no significant difference was observed between hyperprolactinemic rats and control rats, at the dose of 2 mg/kg i.p. of haloperidol.