The role of dorsal striatal mechanisms in the regulation of apomorphine-induced jaw movements was studied. Jaw movements induced by apomorphine (0.2 mg/kg i.v.) were potentiated by quinpirole (10 micrograms/0.2 microliter) injected into the dorsal part of the striatum 10 min before apomorphine. Quinpirole injection into the ventral part of the striatum did not affect the effects of apomorphine. the quinpirole-induced potentiation in the dorsal striatum was prevented by l-sulpiride (25 ng), nemonapride (1 microgram), SCH23390 (1 microgram) or methylscopolamine (1 microgram), but not muscimol (50 ng), co-administered with quinpirole. Injection of these drugs alone 10 min before apomorphine failed to alter the effects of apomorphine. l-Sulpiride (25 ng) injected into the dorsal striatum 60 min before apomorphine increased the frequency of jaw movements induced by apomorphine (0.2 mg/kg). The l-sulpiride-induced potentiation was prevented by methylscopolamine (0.1 microgram) or l-sulpiride (25 ng) injected into the dorsal striatum 10 min before apomorphine; we had already found that this potentiation was also blocked by SCH23390. It is suggested that a synergistic dopamine D1/D2 receptor interaction underlies both the quick-onset potentiation by quinpirole and the delayed-onset potentiation by l-sulpiride.