Ultraviolet B (UVB)-irradiated human keratinocytes and human endothelial cells release nitrogen oxides, i.e. nitric oxide (NO). S-nitrosothiols, hydroxylamine (H2NOH) as well as ammonia (NH3) formed from L-arginine. Generation of these compounds was time and concentration-dependent and decreased by both N-monomethyl-L-arginine (L-NMMA) and N-nitro-L-arginine (L-NA). UVB radiation of the cells resulted in a concomitant increase of soluble guanylate cyclase (sGC) activity which was inhibited by L-NMMA and L-NA. S-nitrosothiols formed during the irradiation of the cells directly increased purified sGC activity by a mechanism characteristic of release of NO from a carried molecule. UVB-irradiated cells promptly increased thiobarbituric acid reacting substance (TBARS) (estimated as malondialdehyde. MDA) production which were inhibited by desferrioxamine. In in vivo experiments using guinea pigs subjected to UVB radiation, a Protection Factor (PF) of 2.25 +/- 0.75 was calculated when an emulsified cream formulation containing L-NMMA (1% w/w) and L-NA (1% w/w) was applied to their skin. In human volunteers subjected to UVB radiation, a dose-dependent increase of PF was observed. When an emulsified cream formulation containing L-NMMA (1% w/w) and L-NA (1% w/w) was applied to their skin the PF was 2.15 +/- 0.80: by increasing the concentration of L-NMMA (1% w/w) and L-NA (2% w/w) the PF was 4.25 +/- 1.25. The present results indicate that UVB radiation acts as a potent stimulator of human keratinocytes and endothelial cells to release nitrogen oxides that may diffuse out of the keratinocytes and endothelial cells, activating sGC in neighboring smooth muscle cells. This may be a major part of the integrated response of the skin leading to vasodilation and erythema.