Experiments were performed to determine whether exogenous cytidine(5')diphosphocholine (CDP-choline) could modify release of dopamine in the striatum and behavior dependent on dopamine, perhaps by providing supplemental choline for synthesis of acetylcholine. Rats received water (control) or CDP-choline orally (100 mg/kg per day, for 5 days), either alone or before injection with haloperidol (1 mg/kg, i.p.), apomorphine (0.15 mg/kg, s.c.), or both. Stereotyped behavior was measured during the hour after administration of apomorphine; levels of the dopamine metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum were assessed at the end of this period (2 hr after administration of haloperidol). In rats receiving the CDP-choline, the stereotyped behavior observed after injection of apomorphine alone (P less than 0.01), or after haloperidol plus apomorphine (P less than 0.01), was attenuated. The pretreatment with CDP-choline also significantly increased levels of HVA (by 24%) and DOPAC (by 23%) in the striatum over appropriate controls in animals receiving haloperidol, or by 29 and 59% (averaging data for all time points), respectively, in animals receiving haloperidol plus apomorphine. One mechanism by which CDP-choline may affect behavior involves contributing choline to enhance synthesis of acetylcholine.