The expression of endothelin-1 (ET-1) and endothelial constitutive nitric oxide synthase (ecNOS) was assessed in two independent in vitro models: asynchronously differentially proliferating cultures and wounded endothelial cell monolayers. Northern blot analysis of RNA isolated from preconfluent, confluent, and postconfluent cells revealed a fourfold rise in ET-1 mRNA transcripts, whereas levels of ecNOS mRNA transcripts were reduced twofold in proliferating cells. Nuclear run-off analysis demonstrated that increased steady-state ET-1 mRNA content in proliferating cells was mediated, in part, by increased gene transcription. In contrast, ecNOS transcription rates in proliferating cells were not decreased compared with quiescent nonproliferating cells, indicating that mRNA destabilization mediated the decreased ecNOS mRNA levels in proliferating endothelium. Concordant changes in protein expression were documented for both ET-1 and ecNOS. In injured endothelial cell monolayers, in situ cRNA hybridization demonstrated increased mRNA transcript levels for ET-1 in growth fronts of injured endothelial monolayers. These data are taken to indicate that expression of ET-1 and ecNOS is reciprocally regulated in two different models of endothelial cell proliferation.