Because renin and angiotensin I (ANG I) level are high in the renal circulation, the conversion of ANG I is a critical step in the regulation of glomerular hemodynamics. We studied this conversion by investigating the effect of ANG I on intracellular Ca²⁺ concentration ([Ca²⁺]_i) in rat juxtamedullary glomerular afferent and efferent arterioles (AA and EA, respectively). Two types of EA were considered, thin EA and muscular EA, terminating as peritubular capillaries and vasa rectae, respectively. In all arterioles, ANG I elicited [Ca²⁺]_i elevations. Maximal responses of 171 ± 28 (AA), 183 ± 7 (muscular EA), and 78 ± 11 nM (thin EA) (n = 6), similar to those obtained with ANG II, were observed with 100 nM ANG I. The EC₅₀ values were 20 times higher for ANG I than for ANG II in AA (10.2 vs. 0.5) and muscular EA (6.8 vs. 0.4 nM) and 150 times higher in thin EA (15.2 vs. 0.1 nM). ANG I effect was blocked by losartan, indicating that AT₁ receptors were involved. The ANG-converting enyzme (ACE) inhibitor lisinopril inhibited the maximal response to ANG I in AA and muscular EA by 75 ± 9% (n = 13) and 70 ± 7% (n = 13), respectively, but had no effect in thin EA (n = 14). The serine protease inhibitor aprotinin, the chymase inhibitor chymostatin, and the cysteine protease inhibitors E64 and leupeptin had no effect on ANG I action. These data show that ANG I effects are mainly mediated by ACE in AA and muscular EA but not in thin EA. The lisinopril-insensitive response may be related to conversion by unknown enzyme(s) and/or to activation of AT₁ receptors by ANG I.