The purpose of the present study was to characterize the nature of the behavioural response to substance P (SP) infusion into the substantia nigra and to evaluate this response in 6-hydroxydopamine (6-OHDA) caudate lesioned rats. The effects of SP infusions (3 microgram in 1 microliter bilaterally) were assessed in an open field. Two groups of rats were used: one with 6-OHDA lesions in the caudate nucleus, and one with sham lesions. In sham rats, the first infusion produced a strong increase in sterotyped rearing and sniffing, with no concurrent enhancement of locomotion. With three subsequent infusions (made at interval of two days) the rearing response disappeared and a tendency to groom emerged. All SP-induced behavioural stimulation was blocked in the caudate-lesioned rats; an effect of the lesion itself was reduced rearing. These results suggest that the response to SP infusion is mediated through the nigro-striatal dopamine system. The behavioural profiles which emerge after SP infusion into the substantia nigra and ventral tegmental area are compared. In general, the behavioural studies of SP effects support the concept that the A9 and A10 dopamine systems can be behaviourally differentiated.