Exposure to angiotensin-converting-enzyme (ACE) inhibitors has been associated with increased ACE activity in vivo and in vitro. In the current study, we examined the effects of the active site-directed ACE inhibitors lisinopril and captopril on ACE gene expression and activity in cultured porcine pulmonary artery endothelial cells. Exposure of endothelial cells to both lisinopril and captopril was associated with increased ACE mRNA levels and concomitant increases in ACE activity. These effects were both concentration and time dependent. ACE mRNA levels began to increase within 30 min of ACE inhibitor exposure and showed an early peak at 2 h and a higher, delayed peak at 48 h. ACE activity peaked at 24 h. Both ACE mRNA levels and activity were highest during incubation with 100 μM inhibitor. Nuclear runoff assays indicated that 48 h of exposure to 100 μM of either captopril or lisinopril increased ACE gene transcription approximately threefold relative to a tubulin control, a level comparable to the increases in ACE mRNA levels and activity observed during ACE inhibitor exposure. These findings support the hypothesis that ACE gene expression in endothelial cells is stimulated by active site-directed ACE inhibitors in vitro. This provides a molecular mechanism for the observation that plasma and tissue ACE activity in vivo is increased during chronic exposure to ACE inhibitors.