We compared effects of feeding arginine- and/or proline- deficient diets (-Arg, -Pro, and -Arg, Pro) with those of a complete diet (Complete) in rats whose small intestine had been massively resected. After 4 weeks, the rats fed -Arg and -Arg, Pro lost weight (a mean of 28 and 32 g, respectively), whereas those fed Complete and -Pro gained 80 and 58 g, respectively. The average nitrogen balance was about 117,100, -20 and -14 mg/day for Complete, -Pro, -Arg, and -Arg, Pro diets, respectively. The concentration of arginine in skeletal muscle was about 310, 330, 91, and 65 nmol/g for Complete, -Pro, -Arg, and -Arg, Pro, respectively; while plasma arginine concentration averaged 95, 107, 56, and 46 microM, respectively. The weight loss, the negative nitrogen balance, and the markedly reduced arginine concentration in the muscle observed in rats fed -Arg and -Arg, Pro clearly indicate that arginine becomes a strictly essential amino acid in the rats with massive resection of the small intestine. However, sufficient proline can be synthesized from arginine in tissues such as the liver and kidney in the absence of the small intestine. Plasma glutamine, citrulline in the muscle and plasma, urinary excretion of orotic acid and nitrate (to assess nitric oxide formation from arginine) were also measured, and the changes in these metabolites are discussed.