To evaluate effects of acute loading of different proteins on renal function, glomerular filtration rate (GFR), albumin excretion rate (AER), and concentrations of plasma amino acids, 11 normal volunteers and 20 diabetic patients were studied before and after eating 1.0 g/kg body weight of either tuna fish meal or bean curd on separate days. In normal subjects, the mean baseline GFR was 115.8 +/- 9.5 ml/min/1.73 m2, and the mean GFRs after ingestion of tuna fish meal were 134.1 +/- 15.5 (1 hr), 146.2 +/- 18.8 (2 hr), and 157.8 +/- 21.2 (3 hr), respectively. GFR did not significantly increase in normal subjects after ingestion of bean curd. GFR in diabetic patients with normoalbuminuria after ingestion of each protein was similar to the response in normal subjects. In diabetic patients with microalbuminuria, GFR did not significantly increase after ingestion of each protein. In diabetes with macroalbuminuria, GFR decreased after ingestion of tuna fish meal and did not significantly change after intake of bean curd. In both normal subjects and diabetic patients, urinary AER did not increase after each kind of protein loading. Plasma concentrations of alanine, glycine, and arginine, known to induce glomerular hyperfiltration, increased to a greater degree after ingestion of tuna fish meal than after administration of bean curd. These findings suggest that responses of GFR to acute protein loading may differ according to the amino acid composition of the protein ingested and to the stage of diabetic nephropathy.