We have tested the possibility that compensatory adrenal growth is neurally mediated. Small unilateral electrolytic lesions were made in the ventral hypothalamus of young male rats. Subsequently, unilateral adrenalectomy was performed on the side ipsilateral or contralateral to the hypothalamic lesion. Four days later the animals were killed and the remaining adrenal was weighed; the location and extent of the hypothalamic lesions were determined. Lesions placed on the same side as adrenalectomy prevented compensatory growth of the remaining gland (p less than 0.005), while lesions on the side opposite to the removed adrenal did not interfere with compensatory adrenal growth. We conclude from these findings, and from a review of some anatomical studies, that compensatory adrenal growth is effected through a reflex that involves both afferent and efferent neural limbs.