The effect of diencephalic stimulation on adrenal medullary secretion was investigated in pentobarbital-anesthetized rats. Adrenal secretions of epinephrine and norepinephrine were evaluated by collections and fluorometric analyses of adrenal venous blood samples. A significant increase in epinephrine secretion was produced by stimulation of the dorsomedial, ventromedial and lateral hypothalamic areas and the posterior regions of the thalamus. Most of the positive responsive areas in the diencephalon were localized near the dorsomedial hypothalamic nucleus. In contrast, a significant decrease in epinephrine secretion with or without a significant decrease in norepinephrine secretion was observed following stimulation of the preoptic area and lateral hypothalamic region and a significant decrease in norepinephrine secretion following stimulation of the anterior region of the thalamus. These findings may be interpreted as indicative of existences in the diencephalon of excitatory and inhibitory central nervous system mechanisms for the control of the adrenal medullary secretion.