Proteolysis mediated by the ubiquitin‐proteasome system has been implicated in the regulation of programmed cell death. Here we investigated the differential effects of proteasomal inhibitors on the viability of proliferating and quiescent primary endothelial cells in vitro and in vivo. Subconfluent, proliferating cells underwent carbobenzoxy‐L‐isoleucyl‐7‐í‐butyl‐L‐glutamyl‐L‐alanyl‐L‐leucinal (PSI)‐induced apoptosis at low concentrations (EC₅₀=24 nM), whereas at least 340‐fold higher concentrations of PSI were necessary to obtain the same effect in confluent, contact‐inhibited cells. PSI‐mediated cell death could be blocked by a caspase‐3 inhibitor (Ac‐DEVD‐H), but not by a caspase‐1 inhibitor (Ac‐YVAD‐H), suggesting that a caspase‐3‐like enzyme is activated during PSI‐induced apoptosis. When applied to the embryonic chick chorioallantoic membrane, a rapidly expanding tissue, PSI induced massive apoptosis also in vivo. PSI treatment of the CAM led to the formation of areas devoid of blood flow due to the induction of apoptosis in endothelial and other cells and to the collapse of capillaries and first order vessels. Our results demonstrate that proteasomal inhibitors such as PSI may prove effective as novel anti‐angiogenic and anti‐neoplastic substances.—Drexler, H. C. A., Risau, W., Konerding, M. A. Inhibition of proteasome function induces programmed cell death in proliferating endothelial cells. FASEB J. 14, 65–77(2000)