The present study examined the role of D1 and D2 receptors in mediating locomotor activity induced by dopamine (DA) agonists after injection into the nucleus accumbens (Acb). The D1 receptor agonist SKF38393 (as the racemic mixture) induced a dose-related increase in activity when injected bilaterally (1-10 micrograms/side). At a dose of 1 microgram/side, only the R-enantiomer was active. The SKF38393 (10 micrograms/side)-induced activity was antagonized by the D1 receptor antagonist SCH23390 (0.5 mg/kg i.p.), by the D2 receptor antagonist spiperone (0.1 mg/kg, i.p.), but not by the 5-HT2 antagonist ketanserin (1 mg/kg, i.p.). Another D1 agonist, CY208 243, also induced a moderate increase in activity when injected into the Acb (2 and 8 micrograms/side), but this was of much less intensity and of shorter duration than that produced by SKF38393. The D2 receptor agonist quinpirole slightly increased activity when administered into the Acb (0.3-3 micrograms/side), with the magnitude and duration of the response, however, being much less than that produced by SKF38393. The locomotor stimulant effects of SKF38393 (5 micrograms/side), CY208 243 (2 micrograms/side) and quinpirole (1 microgram/side) were blocked by the depletion of catecholamines with reserpine (5 mg/kg s.c., 24 h pretreatment) and alpha-methyl-p-tyrosine (200 mg/kg, i.p.). However, when SKF38393 and quinpirole were injected concurrently into the Acb at doses of 5 and 1 microgram/side respectively, a marked locomotor stimulation occurred in catecholamine-depleted rats. Furthermore, SKF38393 (1 microgram/side) or CY208 243 (2 micrograms/side), injected concurrently with quinpirole (0.3 microgram/side), into the Acb of rats with intact DA stores produced an at least additive effect on locomotor activity. These results suggest that both D1 and D2 receptor stimulation in the Acb is required for the expression of locomotor effects. Furthermore, D1 and D2 receptors in this nucleus appear to interact positively with each other, and may mediate the additive locomotor stimulatory effects induced by concurrent systemic administration of selective D1 and D2 agonists.