The effect of ethanol administration on the in vitro release of alpha-neoendorphin (ANEO) and Met-enkephalin-Arg6-Gly7-Leu8 (MEAGL), peptides derived from prodynorphin and proenkephalin, respectively, was studied in the rat hypothalamus. Single ethanol administration had no effect on the tissue level and potassium-stimulated release of these peptides. Repeated ethanol administration increased the release of ANEO and reduced the release of MEAGL from hypothalamic slices. Two days after cessation of ethanol administration the release of MEAGL returned to control values, whereas the release of ANEO was significantly inhibited. On the other hand, the tissue level of those peptides remained unchanged after repeated ethanol or 2 days after its last administration. The present study shows that repeated treatment with ethanol may lead to an increase in the prodynorphin neurons' sensitivity to the depolarizing effect of potassium. In contrast, the sensitivity of the proenkephalin system to ethanol seems to be inhibited. Thus, ethanol appears to have an opposite effect on the sensitivity of hypothalamic proenkephalin and prodynorphin neurons.