Increasing intravenous doses of two gamma-aminobutyric acid (GABA) agonists, muscimol and 4,5,6,7-tetrahydroisoxazolo-[5,4-c]-pyridin-3-ol (THIP), caused dose-related increases in extracellular, single unit activity of A10 dopaminergic neurons of the ventral tegmental area. Muscimol, the more potent of the two compounds, stimulated firing at doses approximately 15-20 times lower than the doses of THIP required to elicit equivalent excitatory effects. The maximum stimulation achieved was approximately 150 and 140% of the baseline firing rate for muscimol and THIP, respectively.