Previous experiments have demonstrated that cats fed an arginine free purified amino acid diet (-Arg) could be protected from hyperammonemia by the inclusion of ornithine in the diet. Experiments reported here show that diets containing ornithine added at an equimolar (+Orn) or five times equimolar (+5 Orn) to replace arginine in the +Arg basal diet (2.0% Arg . HCl), were incapable of maintenance of body weight. Growing kittens fed a -Arg +Orn and -Arg +5 Orn diets had a daily rate of body weight loss of 6.5 +/- 0.6 and 7.9 +/- 2.2 g, respectively, which is somewhat less than the mean rate of weight loss of kittens fed diets devoid of most of the other amino acids indispensable for the kitten. When citrulline was added to a -Arg diet at an equimolar level to the arginine in the +Arg basal diet, growth rates were equivalent. The mean plasma free ornithine concentration of kittens receiving the -Arg +5 Orn diet was about 33 times higher than when they were fed the +Arg diet. For kittens fed the -Arg + Orn and -Arg +5 Orn diets the mean plasma free arginine was less (but significant only for the -Arg + Orn diet) than that observed for the basal diet. When the -Arg + Cit diet was fed, plasma free arginine concentration was similar and plasma free citrulline was about 18 times higher than when these cats were fed the basal diet.