Pre- and postsynaptic adenosine 5′-triphosphate-sensitive potassium (ATP-K+) currents were studied using whole-cell recordings from substantia nigra zona compacta “principal” neurons in midbrain slices. The GABAA and GABAB receptor-mediated synaptic potentials were unaffected by the ATP-K+ channel inhibitor glibenclamide (30 microM) or by the opener diazoxide (500 microM), indicating that ATP-K+ channels on GABA-ergic terminals are not active, nor can they be activated pharmacologically, under control conditions. However, application of a glucose-free solution to reduce intracellular ATP levels caused a reduction of the GABAB IPSP in all neurons. This was substantially reversed by the sulfonylurea inhibitor tolbutamide (300 microM) in 50% of the neurons tested. The reduction of the GABAB IPSP was a presynaptic effect since postsynaptic hyperpolarizations induced by the GABAB receptor agonist baclofen (10 microM) were unaffected by glucose- free solutions. Diazoxide (500 microM) induced a slowly developing hyperpolarization or outward current in 64% of principal neurons, which was tolbutamide- (100–300 microM) or glibenclamide- (30 microM) sensitive. In contrast, the GABAB receptor agonist baclofen (30 microM) induced a rapid hyperpolarization or outward current in all neurons tested that was unaffected by tolbutamide (300 microM). Although both the diazoxide-induced current and the baclofen-induced current were inhibited by Ba2+ (300 microM), the currents elicited by diazoxide and baclofen summated. The reversal potential for the diazoxide-induced current was also less negative than that for baclofen, which was close to EK. In the presence of intracellular cesium, diazoxide induced a tolbutamide-sensitive inward current in a proportion of neurons, indicating that it has other actions in addition to activating a potassium current. Our results suggest that functional ATP-K+ channels exist both pre- and postsynaptically in the SN, where they modulate the activity of principal neurons. They are different to the potassium channels activated by the GABAB receptor agonist baclofen.