Background: L-arginine ( L-arg) participates in numerous biological functions including urea and nitric oxide synthesis. Sources of L-arg include dietary proteins and endogenous synthesis by argininosuccinate synthetase and argininosuccinate lyase. L-arg is converted to urea by arginase I in the liver and arginase II in the kidney. Normally, the liver fully consumes L-arg for urea generation and does not contribute to its circulating pool. Instead, much of the circulating L-arg is produced by the kidney. If true, plasma L-arg should be severely reduced in chronic renal failure (CRF); however, plasma L-arg is frequently unchanged in CRF. We hypothesized that preservation of plasma L-arg in CRF may be, partly, due to downregulation/inhibition of arginase. Methods: Argininosuccinate synthetase, arginase I and II protein abundance and activity were measured in the liver and kidneys of rats 6 weeks after 5/6 nephrectomy or sham operation. In addition, arginase activity was measured in the presence of different urea concentrations to simulate azotemia in vitro. Results: Arginases I and II protein abundance as well as arginase activity in the liver, measured in the physiological buffer, were similar among the CRF and control groups. However, in vitro experiments simulating a uremic milieu revealed a marked concentration-dependent inhibition of arginase activity by urea in the tissue lysates. CRF had no significant effect on argininosuccinate synthetase abundance in the kidney, liver, spleen or intestine. Conclusions: Although CRF does not change the abundance or intrinsic properties of arginase, the inherent rise in urea concentration inhibits its enzymatic activity. The latter, in turn, attenuates L-arg catabolism and urea production and, thereby, mitigates the fall in plasma L-arg.