Injection of microquantities of morphine into two sites in the hypo-thalamus was found to depress the release of radioiodine from the thyroid gland of rats. These sites lay in the supraoptic or the supramammillary region close to the midline. Injection of 0.9 % NaCl into the supraoptic area caused accelerated thyroid activity. These responses to morphine may represent a dual action of the drug: stimulation of neurons in the caudal hypothalamus that inhibit thyroid stimulating hormone release and depression of neurons in the rostral region that normally activate the pituitary-thyroid axis.