Experiments were performed on male Holtzman albino rats. Stainless steel semimicroelectrodes 60 µ in diameter were implanted stereotaxically in the ventromedial hypothalamus (VMH) and in the pineal body (PB). Six to eight days following this operation, the sensory field potentials from freely behaving, unrestrained rats were averaged. After a control recording was achieved, animals were given injections i.p. with 10, 30 or 50 mg/kg of morphine and recordings were resumed for 45 min. Naloxone was then administered and recordings were monitored during another 45 min period. Three consistent components of the averaged sensory evoked potentials were evaluated. In both structures following the two lower doses of morphine (10 and 30 mg/kg), increased and decreased response amplitudes were observed, while following 50 mg/kg morphine, mainly increased response amplitudes were obtained in both structures. With increased doses, the percentage of responses was increased. During the 45 min of recording after morphine, effects remained consistent. Differences in the percentage and the direction of the responses were observed between the VMH and the PB. Naloxone (1 mg/kg) reversed the morphine effect in the two structures, both the increased and decreased responses. The possible interactions between morphine and endocrine function of the two structures are discussed.