The effect of intranigral injections of stable substance P analogs with antagonist properties, namely [D-Pro2, D-Trp7,9]-substance P and [D-Pro2, D-Phe7, D-Trp9]-substance P, on the activity of the nigrostriatal dopaminergic pathway was studied. Dopaminergic neural activity was evaluated by measuring changes in striatal tyrosine hydroxylase activity in response to systemic treatment with haloperidol. When injected into the substantia nigra of normal rats, neither of the two substance P analogs influenced the Vmax of striatal tyrosine hydroxylase or the affinity of the enzyme for its pteridine cofactor. However, when applied into the substantia nigra 10 min before systemic haloperidol, 20 micrograms of either substance P analog was able to prevent the haloperidol-induced activation of striatal tyrosine hydroxylase. The effect was not altered by concurrent microinjection of the GABA-antagonist bicuculline. These results suggests that nigral substance P plays an important role in the activation of nigrostriatal dopamine neurons produced by haloperidol. Thus, both substance P and GABA may reciprocally and independently regulate the activity of nigrostriatal dopamine neurons in response to changes in dopamine receptor activity.