Previous studies suggested that increased blood levels of, or increased tissue sensitivity to, glucocorticoids may contribute to catabolism in acute uremia. To examine this possibility we determined urea nitrogen (urea-N) appearance, plasma levels of Nl-methylhistidine and the activity of the alkaline myofibrillar proteinase in acutely uremic rats with and without treatment with RU 38486, a selective antiglucocorticoid. Forty-eight hours after bilateral nephrectomy, the rats had markedly elevated serum levels of urea-N, creatinine, potassium and phosphorus. In uremic rats receiving RU 38486, comparable levels of serum creatinine were found, but the serum levels of urea-N (221 ± 4 vs. 259 ± 5 mg/dl) and phosphorus (6.5 ± 0.3 vs. 8.5 ± 0.4 mmol/l) were significantly decreased as compared to uremic animals without RU 38486. In comparison to sham-operated rats, urea-N appearance (net urea production) was increased by 56% 48 h after bilateral nephrectomy. This increment was almost completely reversed in uremic animals receiving the antiglucocorticoid. In untreated uremic rats, plasma levels of Ntethylhistidine were 10.3 ± 0.9 μg/dl, whereas the administration of RU 38486 caused a significant decline in the levels of this amino acid (7.6 ± 0.5 μg/dl). This reduction in N’-methylhistidine was associated with a concomitant decrease of myofibrillar proteinase activity in muscle tissue homogenates. Compared to sham-operated animals, this proteinase activity was increased by 30% in uremic rats, but was normal in those given RU 38486. Taken together, these data support the view that in acute uremia accelerated ureagenesis occurs, while enhanced muscle protein breakdown, owing to an increment in myofibrillar proteinase activity, provides the necessary amino acid precursors. Furthermore, the activity of this proteinase, and therefore the rate of muscle protein breakdown, strongly depend on the presence or absence of glucocorticoids