1. The effects of (+)-amphetamine (1-300 microM) applied by superfusion, on substantia nigra zona compacta neurones were studied in slices of the rat mesencephalon with intracellular recording techniques (current and voltage clamp mode). 2. Amphetamine inhibited the action potential discharge, hyperpolarized the cells and increased the membrane conductance. Under voltage clamp it produced an outward current. 3. The amphetamine outward current reversed at potentials negative to -100 mV (2.5 mM extracellular potassium) and was reduced by barium, (1 mM) suggesting an increased potassium conductance. 4. In slices treated with tetrodotoxin and calcium-free solution and added cobalt, amphetamine was still effective. 5. (-)-Sulpiride, (1 microM) a D2 antagonist, blocked the effects of both amphetamine and dopamine. 6. In slices treated for more than 3 h with 30 microM alpha-methyl-DL-tyrosine, an agent known to block the synthesis of dopamine, the action of amphetamine was attenuated, while dopamine was still effective. 7. Responses to applied dopamine were potentiated by cocaine (10 microM) but unaffected by amphetamine (1-3 microM). 8. It is suggested that the inhibition of substantia nigra compacta neurones by amphetamine is mainly due to the release of endogenous dopamine.