This study was designed to examine the ability of naloxone to antagonize the inhibition of neurons in the globus pallidus produced by electrical stimulation of the caudate nucleus. In rats anesthetized with chloral hydrate, bipolar stimulating electrodes were placed in the head of the caudate and seven-barrel micropipettes were utilized for recording extracellular activity of globus pallidus cells as well as for microiontophoretic application of experimental drugs. In most globus pallidus cells which were inhibited by caudate stimulation, application of naloxone (at currents which antagonized morphine-elicited depression) attenuated the caudate-induced effect. Naloxone did not antagonize depression of globus pallidus neurons induced by gamma-aminobutyrate. The results indicate that caudate stimulation causes the release of endogenous enkephalins which act to suppress neuronal activity in the globus pallidus. Thus, this study supports biochemical and histochemical studies which provide evidence for an enkephalinergic component in the striatopallidal projection.