Administration of 6-hydroxydopamine to rats did not alter significantly the corticoid circadian rhythm. Adrenal and serum corticoids were low in the morning and high in the evening in both 6-hydroxydopamine-treated rats and in controls. Whole brain norepinephrine and dopamine in 6- hydroxydopamine-treated animals were decreased to 28.7% and 53.4% of controls, respectively. In another set of rats, hypothalamic biogenic amine depletion was compared with depletion in the rest of the brain in control and drug-treated rats. Hypothalamic and remaining brain norepinephrine were lowered to 38.0% and 39.5%, respectively, of control levels in treated rats. However hypothalamic dopamine was not lowered by drug treatment, although 6-hydroxydopamine decreased dopamine levels in the rest of the brain to 52% of controls. The results show that the circadian corticoid rhythm persists following marked norepinephrine depletion in the hypothalamus as well as in the rest of the brain. Hypothalamic dopamine is not reduced but an approximately 50% reduction of dopamine occurs in the rest of the brain. The results are most compatible with a dopaminemediated central inhibitory system for ACTH at the hypothalamic level but do not exclude the possibility that the remaining noradrenergic fibers may be involved as well. (Endocrinology92: 611, 1973)