The postoperative increase in hepatic conversion of amino nitrogen to urea nitrogen seems to be a primary cause of post-surgical catabolism. The importance of glucocorticosteroids for the spontaneous urea nitrogen synthesis rate (UNSR) and for the maximally amino acid-stimulated capacity of urea nitrogen synthesis (CUNS) was investigated 3 and 24 h postoperatively, respectively, in hysterectomized rats. Corticosteroid effects were neutralized by glucocorticoid receptor blockade by the pharmacological analogue RU486. Hysterectomy doubled UNSR from 3.16 +/- 0.20 to 6.12 +/- 0.27 mumol (per min per 100 g body weight) after 3 h (P less than 0.01) and increased CUNS by 40% from 7.47 +/- 0.30 to 10.29 +/- 0.41 mumol (per min per 100 g body weight) after 24 h (P less than 0.01). These changes were both normalized by the receptor blockade. Hysterectomy decreased total blood alpha-amino nitrogen concentration by 25% from 3.4 +/- 0.2 to 2.6 +/- 0.2 mmol l-1 (P less than 0.05) 3 h after surgery, which was normalized by glucocorticoid receptor blockade. Hysterectomized rats lost 10 +/- 1 g the first 24 h after surgery. The blockade reduced the weight loss to 6 +/- 1 g body weight (P less than 0.05) without changing food intake. The results indicate that glucocorticoid action plays a major role in the postoperative increase in hepatic amino nitrogen conversion.