Microdialysis combined with sensitive radioimmunoassays was used to measure the in vivo release of substance P (SP) and neurokinin A (NKA) in the rat substantia nigra. The effect of acute and subchronic haloperidol treatment (0.5 mg/kg) on the basal and potassium-evoked release was studied. No significant effect was observed after a single injection. However, pretreatment with haloperidol for 10 days decreased the potassium-induced release of SP and NKA by 25 and 27%, respectively. The basal overflow of NKA was reduced by 29%, while no significant effect could be seen on the basal release of SP.