The role of renal functional reserve (RFR; increase in plasma flow and glomerular filtration rate in response to protein loading) as an indicator of increased glomerular hydrostatic pressure and flow was evaluated in recent-onset poorly controlled diabetic rats. Streptozocin-induced diabetic (STZ-D) rats were studied with micropuncture (MP) technique after 10-15 days of diabetes (daily blood glucose level 15.3-18 mmol). We also studied STZ-D rats treated with the converting-enzyme inhibitor (CEI) enalapril or the angiotensin II (ANG II) receptor antagonist DuP 753 (DuP) for 3 days before MP. Nondiabetic rats (NOR) served as controls. Glomerular hemodynamics and proximal tubular reabsorption were measured in the control period and during intravenous glycine infusion. In NOR rats, glycine increased single-nephron plasma flow (SNPF) and single-nephron glomerular filtration rate (SNGFR). Although STZ-D rats did not exhibit hyperfiltration, SNGFR and SNPF were not modified by glycine, defining loss of RFR. CEI rats responded to glycine with an increase in SNGFR due to a rise in SNPF and a rise in the ultrafiltration coefficient. Interestingly, loss of RFR in STZ-D rats was associated with a decrease in absolute proximal reabsorption. The decrease in absolute proximal reabsorption was corrected by both CEI and DuP, although glomerular vasodilation was restored only in the CEI group. In conclusion, at the early stage of diabetes mellitus, loss of RFR does not detect hyperfiltration, but rather the presence of a tubular alteration probably dependent on ANG II.(ABSTRACT TRUNCATED AT 250 WORDS)