Abstract The action of iontophoretically applied metenkephalin and leu-enkephalin on the unit activity of hypothalamic neurons was studied in the urethane anaesthetized female guinea pig. Two neuron types were identified by median eminence electrical stimulation: antidromically activated neurons, called tuberoinfundibular neurons and unidentified neurons. The two enkephalins inhibited discharge of tuberoinfundibular neurons with similar way whereas some neurons were only responsive to one enkephalin. Some inhibitions induced by enkephalins were reversibly antagonized by prior or concurrent iontophoretic administration of naloxone. The unidentified neurons tested with enkephalins displayed similar inhibitory responses. These results suggest that an enkephalinergic innervation of neurons projecting towards the median eminence can be involved, through different opiate receptors in the control of pituitary function by opioid peptides.