delta-N-(phosphonacetyl)-L-ornithine (PALO) is a powerful and specific inhibitor of ornithine transcarbamylase (1), but it does not readily enter intact cells and therefore does not inhibit citrulline synthesis in intact liver or intestine. We have used the glycylglycine derivative of PALO (Gly-Gly-PALO) to evaluate the importance of intestinal citrulline synthesis in supplying arginine for growth. We have shown that the peptide derivative of PALO is selectively taken up by gut cells via the peptide permease and is released intracellularly as the free inhibitor. When administered in drinking water to 6-wk-old rat pups on an arginine-deficient diet, serum citrulline was reduced from 85 +/- 4.2 to 44 +/- 2.2 microM and arginine from 240.1 +/- 19.0 to 52.1 +/- 4.1 microM. Ornithine increased from 100 +/- 6.2 to 273.5 +/- 21.3 microM. Addition of 0.1 mM Gly-Gly-PALO to drinking water caused a rapid and complete inhibition of growth in rats on arginine-deficient diets, and this growth inhibition could be partially prevented by simultaneous administration of 1% (wt/wt) arginine to the diet and completely prevented with 1% (wt/wt) citrulline. The specificity of the effects of Gly-Gly-PALO on intestinal citrulline synthesis was shown by the inability of the drug to be taken up or to inhibit citrulline synthesis in isolated rat hepatocytes, and the oral administration of the drug had no effect on serum ammonia concentrations. The relative importance of endogenous synthesis of arginine compared with dietary arginine for growth was shown by the ability of Gly-Gly-PALO to inhibit the growth of rats maintained on standard laboratory chow containing normal levels of arginine.