The effect of single and chronic ACTH administration on serum levels of the corticosteroid-binding globulin (CBG) and pituitary-adrenal (PA) responsiveness to acute immobilization (IMO) stress was studied in adult Sprague-Dawley rats. Single ACTH administration significantly reduced CBG levels but did not alter PA response to acute IMO. Chronic ACTH administration caused a greater fall in CBG than single ACTH administration and blunted PA response to IMO. The effect of chronic ACTH administration on CGB levels recovered 2 days after the last administration, but the ACTH response to IMO was normal only by day 7 after the last ACTH injection. The present data indicate that ACTH administration to rats reduced CBG levels and impaired PA response to acute stress, but impaired PA responsiveness cannot be solely attributed to changes in CBG.