The effects of morphine administration on concentrations of epinephrine, norepinephrine and dopamine were examined in the rat brain. Morphine injection reduced the epinephrine level only in the hypothalamus, while the norepinephrine level was reduced in the hypothalamus, medulla, and locus coeruleus. The dopamine concentration was elevated in all regions examined. These changes were blocked by administration of naloxone. Repeated injection of morphine for 14 days did not affect any catecholamine level. In naloxone-induced withdrawal, epinephrine was most markedly depleted in hypothalamus. These observations suggest that the epinephrine level in hypothalamus is affected by morphine acting on opioid receptors.