Previous studies demonstrated that the proximal tubule (PT) responds to isolated increases in basolateral (BL) or "bath" [CO₂] by increasing the HCO-₃ reabsorption rate (JHCO₃). Blockade of the rabbit apical AT₁ receptor or knockout of the mouse AT_1A receptor eliminates these effects, demonstrating a requirement for luminal ANG II that the PT itself synthesizes. In the present study, we examined the effects of the ACE inhibitor lisinopril on JHCO₃ in isolated perfused rabbit PTs (S2 segment), using out-of-equilibrium solutions to make isolated changes in [CO₂]_BL at a fixed [HCO-₃]_BL of 22 mM and fixed [pH]_BL of 7.4. Adding 60 nM or 240 nM lisinopril (in-vitro K_i: 0.5 - 1.2 nM) to the lumen had no effect. These results are not consistent with the hypothesis that the PT secretes either angiotensinogen or ANG I. However, adding 60 nM basolateral lisinopril significantly decreased JHCO₃ at a [CO₂]_BL of 20%. Moreover, 240 nM basolateral lisinopril decreased baseline (i.e., at 5% CO₂) JHCO₃ by half and completely eliminated the response to altering [CO₂]_BL from 0 to 20%, but left intact the stimulatory effect of 10-¹¹ M basolateral ANG II. At extremely high concentrations (i.e., 100 µM), luminal lisinopril replicated the effects of 240 nM basolateral lisinopril. Our data are consistent with the hypothesis that lisinopril readily crosses the basolateral (but not apical) membrane to block ACE in a vesicular compartment. We conclude that the isolated PT predominantly secretes preformed ANG II, rather than angiotensinogen or ANG I.