We have studied the renal hemodynamic effects of short-term protein restriction and short-term protein loading in 12 healthy volunteers, who adhered to a low protein diet (0.6 g/kg/day) for three weeks and then switched to a high protein diet (1.8 g/kg/day) for another three weeks. Baseline glomerular filtration rate (GFR) was not influenced by dietary protein intake. Effective renal plasma flow (ERPF), however, was significantly lower on the low protein diet (491 +/- 23 ml/min; mean +/- SEM) as compared to values on the high protein diet (538 +/- 19 ml/min; p less than 0.01). The increase of GFR after a meat meal (providing 1.8 g protein/kg body weight) was higher on the high protein diet (GFR: +23.6 +/- 5.1 ml/min) than on the low protein diet (GFR: +13.0 +/- 2.4 ml/min; p less than 0.05). We conclude that three weeks of dietary protein restriction do not decrease glomerular filtration rate and do not enhance the renal response to a meat meal. Therefore, it seems doubtful that dietary protein restriction decreases glomerular capillary pressure through vasoconstriction of the afferent glomerular arteriole. Furthermore, measurement of the renal hemodynamic response to an acute protein load seems of doubtful physiological significance.