The effects of dietary protein content on diurnal variations in plasma concentrations of neutral and basic amino acids, and on red blood cell levels of acidic amino acids, were studied in seven normal humans. The subjects consumed, on three consecutive 3-day periods, diets containing 0, 75, or 150 g of egg protein per day; blood was collected at 4-h intervals on the 2nd and 3rd days of each diet. For each of the large neutral amino acids (LNAA; isoleucine, leucine, tyrosine, phenylalanine, methionine, valine, and tryptophan) significant correlations were observed between its plasma levels and the protein content of the diet; highest levels were noted after consumption of the 150-g protein diet, and lowest values after the O-g protein diet. For each LNAA, except tryptophan, "fed" values (ie, those at 3 PM and 7 PM) were decreased relative to "fasting" values (those at 3 AM and 7 AM) after consumption of the O-g protein-free diet, but increased after consumption of the 150-g protein diet. Threonine, serine, and proline behaved like the LNAA: in contrast, glycine and alanine rose after protein-free meals and fell with the high-protein diet. The basic amino acids, lysine, arginine, and histidine tended to respond like the LNAA to variations in dietary protein content. Red blood cell concentrations of glutamate tended to vary inversely with the protein content of the diet, while no relationship was noted between red blood cell aspartate and dietary protein content. Food-induced changes in plasma LNAA have been found to affect brain levels of amino acids that are neurotransmitter precursors, as well as the syntheses of the transmitters themselves.(ABSTRACT TRUNCATED AT 250 WORDS)