The spontaneous firing rate of nucleus raphe magnus (NRM) neurons increased, decreased or were not altered in Wistar rats after various doses of morphine were administered intravenously. Dose-related changes were observed only in neurons which displayed increased firing frequency. Naloxone antagonized the morphine-induced effect more frequently in the excited neuron than in the inhibited one. These results suggest that morphine induced increases in discharge frequency is the more specific pharmacological effect, and that only the excited neuron might participate directly in the descending analgesic system of morphine.