Adrenocortical function was studied in rats after shampinealectomy, pinealectomy, or immunization against melatonin and N-acetylserotonin (NAS). Pineals were stimulated to increase melatonin synthesis by blinding or exposure to 23 h of darkness daily. Blinding elevated morning corticosterone levels without altering the corticosterone response to novelty stimulation. Among blinded animals, pinealectomy partially reversed and immunization completely reversed the elevations in morning steroid levels. Exposure to short daily photoperiods flattened the diurnal corticosterone rhythm. Pinealectomy did not affect morning corticosterone levels but reduced evening corticosterone levels. More importantly, immunization resulted in reduced corticosterone levels throughout the diurnal cycle. These findings suggest that melatonin and (or) NAS may be involved in the regulation of resting diurnal adrenocortical function.