Nitric oxide (NO) plays a pivotal role in ultraviolet radiation-induced inflammation in human skin. We had earlier reported on the inducible nitric oxide synthase (iNOS) inducing activity of UVA radiation. We now demonstrate that UVB-exposure induces expression of the iNOS in vessel endothelia of normal human skin and in cultured human dermal endothelial cells (HUDEC), although by a molecular mechanism different from UVA-mediated induction. With HUDEC, UVB induces iNOS expression and leads to significant enzyme activities, although at app. 5-fold lower levels than can be achieved with proinflammatory cytokines. In contrast to our earlier observation with UVA, cytokine-challenge combined with simultaneous UVB-exposure had no additive effects on iNOS expression nor activity. Interestingly, a time-delay between UVB-irradiation and cytokine-challenge enhances endothelial iNOS enzyme activity 2.5-fold over cytokines activation only. This time-dependent effect strongly correlates with UVB-induced endothelial TNF-alpha expression. In HUDEC addition of TNF-alpha results in enhanced expression of the inducible arginine transporter system CAT-2 essential for substrate supply and thus iNOS activity. In summary, UVB induces iNOS mRNA and enzyme activity in HUDEC. Moreover, UVB augments CAT-2 expression through a TNF-alpha- dependent mechanism which essentially contributes to increased iNOS activity.