Studies were conducted to determine whether rainbow trout fingerlings possess the ability to synthesize arginine via the urea cycle. Several urea cycle enzymes were detected in trout tissues. An experiment was conducted to determine whether the enzymes increase in response to starvation or in response to dietary protein level (0, 30, 40, 50% protein). Although some effects were observed, they did not appear to be consistent with the function of the urea cycle as a mechanism of detoxifying ammonia in the fish. The activities of kidney arginase and liver and muscle carbamoyl phosphate synthetase (CPS) were higher (P < 0.05) when protein was omitted from the diet (P < 0.05) than when it was present but were unaffected by protein level otherwise. The activities of liver arginase and kidney and muscle CPS and ornithine transcarbamoylase (OTC) were higher (P < 0.05) in starved fish than in fish that received adequate levels of protein. Liver CPS and OTC were lower in starved fish than in fish fed 30% protein. L-[l-¹⁴C]ornithine hydrochloride and L-[carbamoyl-¹⁴C]citrulline, injected intraperitoneally, were incorporated into tissue arginine, a finding consistent with arginine biosynthesis via the urea cycle. When one-half of dietary arginine was replaced by equimolar amounts of glutamic acid, ornithine or citrulline, glutamic acid markedly reduced growth (P < 0.05), whereas growth was depressed only slightly by ornithine (P < 0.05) and not depressed by citrulline (P > 0.05). We conclude that trout have a urea cyle that provides for potential arginine biosynthesis.