Abstract The effects of iontophoretically applied morphine and naloxone were examined on 62 neurons in the preoptic/anterior hypothalamus (POAH) of urethane-anesthetized Sprague-Dawley rats. Thirty-four temperature-insensitive cells responded variably to morphine application. Four cells were excited, 19 inhibited and 11 remained unaffected. Morphine increased the firing rate 12 of 20 warm-sensitive cells. The other 8 were unaffected. None were inhibited. The spontaneous activity of 7 cold-sensitive cells was decreased by morphine application. One cell was unaffected and none were excited. Naloxone ( ≤ 15 nA) antagonized morphine's excitatoory effects on warm-sensitive cells and its inhibitory effects on cold-sensitive cells. Naloxone failed to antagonize any of morphine's actions on temperature-insensitive cells. Our results demonstrate that morphine excited warm-sensitive cells, which are assumed to mediate heat-dissipation responses, and inhibited cold-sensitive cells, which are assumed to mediate heat-gain responses. These actions parallel morphine's hypothermic action in the intact animal and, therefore, suggest that morphine lowers body temperature by exerting a coherent action on POAH warm- and cold-sensitive neurons. Since these effects were antagonized by naloxone, the action of morphine on warm- and cold-sensitive cells seems to be mediated by an opiate receptor.