Renal functional reserve (renal response to protein loading, RFR) has been suggested as a method to verify the presence of hyperfiltration. This study was designed to evaluate the role of RFR as an indicator of increased glomerular capillary hydrostatic pressure in short-term treated and untreated rats with two-kidney, one-clip Goldblatt hypertension. One month after placing a silver clip, micropuncture studies were performed on the unclipped kidney. Normal rats and three groups of clipped rats [untreated group (HYP), a group treated with captopril (CEI) and a group treated with verapamil (VER) 5 days before the micropuncture studies] were studied. Glomerular hemodynamics and proximal tubular reabsorption were measured in control period and during intravenous administration of glycine (G). In normal rats, G produced afferent and efferent dilation, increases in single nephron plasma flow (SNPF) and single nephron glomerular filtration rate (SNGFR) of 24%. Systemic hypertension in HYP rats was associated with increases in transcapillary pressure gradient (DELTA-P) and SNGFR. In this hyperfiltration state, infusion of G did not modify SNGFR of SNPF defining loss of RFR. The antihypertensive treatment was equally effective in normalizing MAP and DELTA-P in CEI and VER, but only CEI rats responded to G with a 20% increase in SNGFR due to an increase in DELTA-P. The most striking findings were that loss of RFR in both HYP and VER rats was associated with a significant decrease in absolute and proximal fractional reabsorption. We conclude that: 1) loss of RFR is not always associated with the presence of glomerular hypertension; and 2) decreases in proximal tubular reabsorption strongly correlate with loss of RFR, suggesting an important role for tubular function in the glomerular hemodynamic response to intravenous amino acid infusion.