1 The mechanism of human sunburn is poorly understood but its characteristic features include the development of erythema. In this study we attempted to determine whether human keratinocytes possess a nitric oxide (NO) synthase (NOS), if this enzyme could be activated to release NO following exposure to ultraviolet B (u.v.B) and to define whether this photo-induced response could be involved in the pathogenesis of sunburn erythema. 2 Treatment of human keratinocytes with various doses of u.v.B (290-320 nm) radiation (up to 100 mJ cm-2) resulted in a dose-dependent release of NO and cyclic GMP production that was reduced by NG-monomethyl-L-arginine (L-NMMA). 3 u.v.B irradiation of keratinocyte cytosol at varying doses (up to 50 mJ cm-2), resulted in a gradual rise in NO production, with a concomitant increase in soluble guanylate cyclase activity (sGC). 4 NOS isolated from the keratinocyte cytosol was constitutively expressed and was dependent on NADPH, Ca2+/calmodulin, tetrahydrobiopterin and flavins. 5 In reconstitution experiments, when purified NOS was added to purified sGC, both isolated from keratinocyte cytosol, a four fold increase in cyclic GMP was observed. The GMP was increased by NO synthesized following u.v.B radiation (up to 20 mJ cm-2) of NOS.