We have recently isolated three opioid peptides, i.e., Met- and Leu-enkephalins and Met-enkephalin-Arg6-Phe7, from the avian brain. In the present study, therefore, effects of these endogenous opioid peptides on the electrical activity of preoptic and hypothalamic neurons of the adult male Japanese quail were examined using a brain slice preparation. All of the three opioid peptides inhibited the spontaneous firing activities of subsets of neurons in the preoptic area and the paraventricular nucleus in the hypothalamus. Threshold concentrations for the inhibitory action were between 10(-7) and 10(-6) M in Met- and Leu-enkephalins and approximately 10(-6) M in Met-enkephalin-Arg6-Phe7, respectively. In a few cells in these brain areas, however, Leu-enkephalin rather potentiated the spontaneous activities, resulting in an increase of firing rates or a decrease of interburst intervals. The inhibitory effect of Met-enkephalin was completely blocked by naloxone, an opioid receptor antagonist, but not affected by bicuculline, a gamma-aminobutyric acid A (GABAA) receptor antagonist. These results suggest that there are functional opiate receptors in subsets of preoptic/hypothalamic neurons and that one of their main physiological functions in these areas is an inhibition of neuronal activities. Because these brain regions are considered to be involved with the regulation of a variety of male reproductive behaviors, opioid peptides may regulate some reproductive behavior through the mechanism that provokes such an inhibition.