The ability of apomorphine (APO) and haloperidol (HAL) to affect the spontaneous release of newly synthesized 3H-DA in the striatum was studied in halothane anesthetized, gallamine paralyzed, awake restrained and freely moving rats. The striatum was continuously superfused through a push-pull cannula with a physiological medium enriched in 3H-tyrosine. Basal levels of 3H-DA release were different in the four experimental models: highest in halothane anesthetized rats, intermediate in awake restrained and gallamine treated rats and lowest in freely moving rats. In all experimental models IV or SC injection of APO (1 mg/kg) inhibited the release of 3H-DA (30-50%) from 15 to 90 min following its administration. In awake restrained and freely moving rats, stereotyped behaviour was observed for one hour following the APO injection. In halothane anesthetized rats the inhibitory effect of APO on 3H-DA release was prevented by pretreatment with HAL (2 mg/kg IV). Injection of HAL (2 mg/kg IV or SC) failed to enhance the release of 3H-DA in anesthetized and awake restrained rats, whilst a long-lasting increase in 3H-DA release was observed in gallamine treated and freely moving animals (55% and 120% respectively). However, catalepsy was observed in both restrained and freely moving rats. It is concluded that the modifications of 3H-DA release produced by HAL but not those produced by APO are dependent on the experimental model used, a fact possibly related to the different sites of action of these two drugs.