1. Intracellular recordings were made from 193 substantia nigra zona compacta neurones in slices of rat mesencephalon. All cells were hyperpolarized by baclofen; this was accompanied by a fall in input resistance. Cells voltage clamped at -60 mV showed an outward current associated with a conductance increase in response to baclofen. The baclofen effects were concentration dependent (effective range 0.3-30 microM); the concentration producing half the maximal effect was 1.5 microM. (-)-Baclofen was 300-700 times more potent than (+)-baclofen. 2. The potential change or membrane current caused by baclofen reversed polarity at -108.8 +/- 1.1 mV (n = 10) when the potassium ion concentration was 2.5 mM, -96.0 +/- 2.8 mV (n = 3) in 4.5 mM-potassium and -76.6 +/- 1.7 mV (n = 5) in 10.5 mM-potassium. The relationship between reversal potential and potassium concentration conformed to the Nernst equation. 3. Dopamine was also applied to 119 of these neurones; all exhibited either a hyperpolarization or an outward current. 4. Baclofen and dopamine outward currents were reduced reversibly by barium (100-300 microM) and tetraethylammonium (10 mM). Superfusion for 5-10 min with solutions presumed to block calcium currents reduced, but did not abolish, responses to baclofen. The effect of baclofen persisted in tetrodotoxin (1 microM). 5. Superfusion of gamma-aminobutyric acid (GABA, 0.3-3 mM) caused either membrane depolarization or hyperpolarization, accompanied by a fall in input resistance. The depolarization was mimicked by muscimol (10 microM) and blocked by bicuculline methiodide (10-100 microM); the hyperpolarization was resistant to bicuculline. Nipecotic acid (500 microM) enhanced the effect of GABA, but was without effect upon the actions of muscimol and baclofen. 6. The effect of dopamine was enhanced by cocaine (10 microM) and antagonized by (-)-sulpiride (0.1-1 microM), whereas the actions of baclofen were unaffected by cocaine or (-)-sulpiride. The maximum outward current produced by dopamine was approximately half that produced by baclofen. 7. Outward currents produced by dopamine were reversibly occluded by maximal outward currents caused by baclofen. 8. Baclofen and dopamine hyperpolarizations were unaffected by intracerebroventricular injection of animals with pertussis toxin. 9. Cells impaled with electrodes containing guanosine 5'-O-(3-thiotriphosphate) (1 mM) were hyperpolarized by both baclofen and dopamine, but the membrane potential did not fully return to its original level when agonist application was discontinued. 10. It is concluded that activation of both dopamine D2 and GABAB receptors may increase the same potassium conductance.(ABSTRACT TRUNCATED AT 400 WORDS)