Department of Physiology, Abramsky Hall, Queen's University, Kingston, Ontario, Canada, K7L 3N6 (Received 17 July 1978) It is well established that the stress of ether anaesthesia raises the level of prolactin in the circulation of the rat (Wakabayashi, Arimura & Schally, 1971; Krulich, Hefco, Illner & Read, 1974). Morphine or the endogenous morphine-like peptides, endorphins and enkephalins, can stimulate the secretion of prolactin in vivo (Ojeda, Harms & McCann, 1974; Martin, Audet & Saunders, 1975; Bruni, Van Vugt, Marshall & Meites, 1977; Rivier, Vale, Ling, Brown & Guillemin, 1977; Chihara, Arimura, Coy & Schally, 1978); naloxone, however, blocks the stimulatory effects of these substances (Bruni et al. 1977; Rivier et al. 1977). It was recently demonstrated that naloxone inhibits the secretion of prolactin induced by stress due to heat or immobilization (Van Vugt, Bruni & Meites, 1978). The restraintinduced stress produced an immediate increase in the concentration of prolactin