The effects of opioid peptides on paraventricular neurones were investigated by using intracellular recordings from hypothalamic slices of the guinea pig. Forty-eight out of 128 neurones were hyperpolarized by DAGO, a synthetic structural analogue of enkephalin selective for mu-receptors. This effect was concentration-dependent and reversibly suppressed by naloxone. DPLPE, a selective delta-agonist, and U-50,488, a selective kappa-agonist, had no effect. The localization and the size of the recorded perikarya were assessed following injection of the fluorescent dye Lucifer yellow. Most of the DAGO-responsive neurones were located within the paraventricular nucleus, some of them in the region of the nucleus which is rich in vasopressin-containing cells, as shown by immunocytochemistry. DAGO-sensitive cells were found among magnocellular as well as parvocellular neurones.