Eight enzymes involved in the conversion of acetylglutamate to arginine in Neurospora crassa were studied. The data indicate that of three enzymes early in the sequence, only the first, acetylglutamate kinase, is a nonorganellar enzyme. The next two, N-acetyl-gamma-glutamyl-phosphate reductase and acetylornithine aminotransferase, are in the mitochondrion, which was previously shown to contain the subsequent enzymes: acetylornithine-glutamate acetyltransferase, ornithine carbamyltransferase, and carbamyl-phosphate synthetase A (arginine specific). The last two enzymes of the pathway, argininosuccinate synthetase and argininosuccinate lyase, were previously shown to be cytosolic. All enzymes but one have low amplitudes or repression. Their levels respond little to arginine excess and are about twofold elevated (threefold for ornithine carbamyltransferase) as a result of arginine limitation in the arg-12-8 strain. No restriction of the incorporation of mitochondrial enzymes into mitochondria could be detected when the levels of these enzymes were elevated. Two enzymes, acetylglutamate kinase and carbamyl-phosphate synthetase A, which initiate the synthesis of the ornithine and guanidino moieties of arginine, respectively, show the lowest specific activities in crude extract. These enzymes display special regulatroy features. Acetylglutamate kinase, which has a typically low amplitude of repression, is subject to feedback inhibition. Carbamyl-phosphate synthetase A is wholly insensitive to arginine or citrulline in vitro or in vivo, but displays a very large amplitude of repression (about 60-fold). It is unique in that it can be almost completely repressed by growth of mycelia in excess arginine. These data suggest that mitochondrial localization may be incompatible with a mechanism of feedback inhibition by a cytosolic effector, arginine. Further, they suggest that the high repressibility of carbamyl-phosphate synthetase A compensates for its feedback insensitivity.