We recently reported that intrarenal vascular AT1 angiotensin II (ANG II) receptors are major determinants of the increased vascular resistance and reactivity to ANG II observed in the kidney of spontaneously hypertensive rats (SHR). We decided to test the hypothesis that, by modifying plasma ANG II levels by inhibiting the ANG II-converting enzyme (ACE) with captopril, we would modify intrarenal ANG II receptors, and therefore the renal vascular response to ANG II. Two approaches were taken: (1) radioligand binding assays were performed on membrane preparations of purified renal microvessels and glomeruli, with displacement of 125I-[Sar-Ile8]-ANG II by specific non-peptide antagonists of AT (losartan) and AT2 (PD 123319): (2) dose-response curves to ANG II on the isolated perfused kidney were studied. Two weeks of captopril treatment significantly reduced blood pressure (BP) and relative heart weight, and increased plasma renin activity. The binding assays showed that renal microvessels and glomeruli expressed a single receptor population (AT1) for ANG II. The density of glomerular AT1 was not modulated by captopril treatment (600 +/- 174 v 573 +/- 97 fmol/mg protein in non-treated and treated SHR respectively); however. AT1 density on the intrarenal arteries increased 3-fold (55 +/- 20 v 154 +/- 30 fmol/mg protein in non-treated and treated SHR respectively. P < 0.05). Experiments with isolated perfused kidneys demonstrated that captopril did not improve the compliance of intrarenal vessels to high flow but increased their reactivity to ANG II (ED50 = 18 nM v 0.5 pM, P < 0.01). We conclude that treatment with an ACE inhibitor increases vascular reactivity to ANG II which may be mediated by an upregulation of renal vascular ANG II receptors.