Glomerular filtration rate, renal plasma flow and urinary albumin excretion rate were measured during insulin-induced euglycaemia in 12 male Type 1 (insulin-dependent) diabetic patients after a 3-week period of low protein diet (45 g/day) or a similar period on unchanged conventional diet (103 g/day). No changes in glycaemic control, indicated by home blood glucose profiles and serum fructosamine concentration, or in arterial pressure, were noted on either diet. On low protein diet, glomerular filtration rate was lower (p less than 0.001) in all patients, but there was no difference in renal plasma flow between low protein diet and normal protein diet; filtration fraction fell significantly on low protein diet (p less than 0.001). Fractional clearance of albumin was also lower (p less than 0.05) on low protein diet. This study suggests that reduction of dietary protein induces, independently of changes in plasma glucose and arterial pressure, modifications in glomerular filtration rate, filtration fraction and fractional clearance of albumin, which may be associated with a beneficial effect on the evolution of diabetic renal disease.