Four experiments were conducted with an arginine (Arg)-deficient, semipurified diet to determine the Arg requirement of the weanling pig. The semipurified diet contained .18% bioavailable Arg as determined by a chick bioassay procedure and, when properly supplemented with Arg, allowed rat growth equal to that obtained with a 19.2% protein corn-soybean meal diet. A preliminary experiment with pigs indicated that the semi-purified diet was markedly deficient in Arg. The Arg requirement of the pig was estimated by adding L-Arg to the semipurified diet such that final dietary Arg levels were .18, .23, .28, .38 and .48% (Exp. 2 and 3) or .48, .58 and .68% (Exp. 4). Weight gain and efficiency of feed utilization were maximized at .48% dietary Arg. Plasma urea-N was increased and plasma NH3-N decreased by .48% Arg compared with lower levels. A slight orotic aciduria was observed in pigs fed Arg levels below .48%. From these results, it is concluded that the Arg requirement of the weanling pig is .48% of the diet, a level twice the current NRC estimate.