The small intestine has been presumed to release citrulline as a precursor for the endogenous arginine synthesis. We studied the effect of intestinal resection and arginine-free diet on rat physiology. We maintained rats with massively resected small intestine (R rats) and those with transected intestines (T rats) on either control or an arginine-free diet. After 4 wk, R rats fed deficient diet [R(-)] lost weight by a mean of 46 g, whereas R rats fed control diet [R(+)] and T rats fed control [T(+)] and deficient diet [T(-)] gained 30-96 g. Average nitrogen balance was 150, 60, 110, and -33 mg/day for T(+), T(-), R(+), and R(-), respectively. The concentrations of arginine in skeletal muscle were 654, 163, 230, and 84 nmol/g, respectively, and those in plasma were 133, 50, 103, and 54 microM, respectively. The concentrations of citrulline in R rats were halved compared with T rats irrespective of diet. We conclude that arginine is synthesized in a small intestine-dependent manner in the rat.