The constitutive calcium/calmodulin-dependent nitric oxide (NO) syn~hasc cxprcsscd in vascular endothclium shams common biochcmjcal and pharmacologic properties with ncuronal NO synthasc. However. rcccnl cloning und molecular charaacrizalion of NO synthase from bovine endothclial cells indicated the existence of a family of constitutivo NO synlhascs. Accordingly, we undertook molecular cloning and scqucncc analysis of human cndothelial NO synthasc. Complemcruary DNA clones prcdicl a protein of 1,203 amino acids sharing 94% identity with the bovine cndothelial protein, but only 60% identity with the rut brain NO synlhasc isolbrm. Northern blot analysis with an endothclial-dcrivcdcDNA identitied a 4.6-4.8 kb mRNA transcript in IWVEC und in situ hybridization localized transcripts to vascular endothelium but not neuronal tissue.