The effects of chronic (14 day) intracerebroventricular infusion of various amounts of ovine corticotropin-releasing factor (oCRF) on the circadian blood corticosterone rhythm in male rats were examined. Control (saline-infused) rats showed distinct blood corticosterone rhythms over 48 h with nadirs at 0900 h and peaks at 2100 h on days 6-7 and 13-14. oCRF at 3 pmol/h did not affect the circadian corticosterone rhythm on these days. When oCRF was infused at a rate of 12 pmol/h, blood corticosterone was increased throughout the 48 h periods. A significant circadian rhythm remained at days 6-7, but continuous infusion for an additional 7 days disrupted the rhythm. Higher doses of oCRF (48 and 240 pmol/h) obliterated the rhythm during both periods; the disruption was characterized by an increase in corticosterone during the lights-on period without a substantial change in the evening maximum. Thus, the blood corticosterone concentration was eventually confined within a narrow range, not exceeding the normal circadian peak, over a wide dose range of centrally administered CRF. Significant effects of oCRF on body and adrenal weight were observed only at the two highest doses used. These findings may provide some insight into the state of the hypothalamic-pituitary-adrenal axis in animals exposed to chronic stress and in patients with depression.