Dietary protein independently modulates albuminuria (U(Alb)V) and albumin synthesis (AlbSyn) in nephrotic rats. While some amino acids are without effect on renal hemodynamics, arginine (Arg) augments renal blood flow and glomerular filtration rate, increases AlbSyn in tissue culture and isolated perfused livers, and could be one specific amino acid causing both decreased glomerular permselectivity and increased AlbSyn. Nephrotic rats were fed 10% casein (LP); 30% casein (HP); 30% casein with the inhibitor of nitric oxide (NO) synthesis N omega-nitro-L-arginine methyl ester (HP + L-NAME); 10% casein supplemented with Arg and amino acids that are Arg precursors of or are derived from Arg (proline, glutamate, and aspartate) in an amount in the increment between 10 and 30% casein (ArgAA); ArgAA supplemented with NH4 acetate to provide a diet isonitrogenous to 30% casein (ArgAA + NH4); or 10% casein plus an incomplete mixture of amino acids (Inc) containing the increment in histidine, phenylalanine, tryptophan, tyrosine, lysine, glycine, alanine, serine, threonine, cysteine, and methionine provided when the diet was changed from 10 to 30% casein. U(Alb)V increased significantly in HP and by a significantly greater amount in HP + L-NAME, but did not change in LP, ArgAA, or ArgAA + NH4. U(Alb)V tended to increase in Inc, was significantly greater than in LP or in ArgAA + NH4, but less than in HP. AlbSyn ([3H]phenylalanine incorporation) was no different in Inc than in HP, and was significantly greater than in either ArgAA + NH4 or LP. Increased AlbSyn results from increased ingestion of one or more of amino acids in Inc, but not from Arg or its precursors or products or from total dietary nitrogen.(ABSTRACT TRUNCATED AT 250 WORDS)