Fifteen days after bilateral lesions of the substantia nigra by local infusion of kainic acid (0.75 microgram) or after intranigral injection of vehicle, rats were administered 0.1, 0.25, 1.0 and 2.5 mg/kg s.c. of apomorphine and the stereotyped items (locomotion, sniffing and gnawing) were recorded on an event-recorder and motility was measured by a photocell apparatus. After low doses of apomorphine (0.1, 0.2 mg/kg), rats lesioned in the substantia nigra with kainic acid showed a degree of stimulation of motility and of sniffing similar to controls; on the other hand, in rats lesioned with kainic acid in the nigra, a dramatic reduction of gnawing and its replacement by sniffing was observed after administration of higher doses of apomorphine (1.0, 2.5 mg/kg). Bilateral infusion of kainic acid (0.75 microgram) into the reticular information, 2.0 mm dorsal to the substantia nigra, had no effect on apomorphine-induced stereotyped behaviour. These results are in agreement with the concept that the substantia nigra, through non-DA pars reticulata neurons, mediates motor and behavioural syndromes of striatal origin.