Previous work has demonstrated a continuous release of citrulline from the small intestine into the circulation. To evaluate the physiologic significance of this process, we have now measured citrulline uptake and release by isolated, perfused livers and, through surgical means and arteriovenous difference measurements, by various organs of the rat in vivo. Intestinally derived citrulline, an end product of glutamine nitrogen metabolism, passes through the liver without appreciable uptake. No significant extraintestinal source of circulating citrulline was found. Contrary to earlier suggestions, the liver releases no citrulline normally, but only when supplied with unphysiological high levels of ornithine and ammonia. Renal citrulline uptake was equivalent to approximately 83% the rate of intestinal release; kidneys, in turn, released arginine equivalent to approximately 75% of the citrulline taken up. Acute experiments in which the intestine, intestine plus liver, or kidneys were excluded from the circulation indicate that additional organs may also participate in citrulline exchange, at least when the circulating citrulline level is abnormal. The intestinal-renal pathway seems to account for a large though still unmeasured portion of the citrulline turnover in the circulation.