Adjuvant-induced arthritis (AA) in Sprague-Dawley rats resulted in a chronic increase in plasma levels of ACTH and corticosterone (B). Joint inflammation became clinically apparent between days 12-15 after injection of adjuvant and reached a peak on day 21, after which time it subsided. In AA animals, plasma ACTH and B levels in the morning (0800-0900 h) on days 7, 8, 9, and 21 were significantly higher than those in control animals (day 0). The corresponding evening ACTH and B levels in AA animals were not significantly different from evening levels in the control animals. Adrenal weight in AA animals was increased on day 21, while thymic weight diminished gradually from days 7-21 postinjection. Development of AA was associated with activation of the hypothalamo-pituitary-adrenal axis, with increased morning ACTH and B levels and abolition of normal diurnal ACTH and B rhythms. This model of chronic inflammatory stress clearly activates the ACTH drive despite increased corticosteroid feedback in the morning, resulting overall in chronically increased B secretion.