The effect of bilateral lesions of the suprachiasmatic nuclei (SCN) on the circadian rhythm in ACTH was studied in rats that were adrenalectomized and implanted with a subcutaneous corticosterone (B) pellet. Rats wee chronically cannulated to allow for repeated blood sampling. In rats with B pellets, bilateral lesions of the SCN eliminated the circadian rise in plasma ACTH seen in sham-lesioned animals. This is consistent with the idea that the SCN stimulate ACTH secretion in the evening.