The effect of the serotonin precursor 5-hydroxytryptophan (5-HTP) on the pituitary-adrenal system of albino rats with neurally isolated medial basal hypothalamus (MBH) was studied. Four weeks after complete deafferentation of the MBH, the serotonin level in the MBH had fallen, but 5-HTP administration to such rats was accompanied by a considerable rise of the amine. Total interruption of neural afferents to the MBH did not prevent a stimulatory effect of 5-HTP on the pituitary-adrenal complex, although no direct action was exerted by 5-HTP on the adrenal cortex: hypophysectomy completely abolished the corticosterone stimulating effect of 5-HTP. The conclusion is drawn that serotoninergic neurons of the hypophysiotropic region of the hypothalamus are involved in a process related to the control of CRF secretion.