Ultraviolet B (UVB) radiation is the main physiological stimulus for human skin pigmentation; however, the molecular mechanisms underlying this process are still unclear. Recently, nitric oxide (NO) and cGMP have been involved in mediation of skin erythema induced by UVB. Therefore, we investigated the role of NO and cGMP in UVB-induced melanogenesis. In this study, we demonstrated that UVB stimulation of melanogenesis was mimicked by exogenous NO donors. Additionally, we showed that NO stimulated cGMP synthesis and that cGMP was also a potent stimulator of melanogenesis. Furthermore, the inhibition of the melanogenic effect of NO by guanylate cyclase inhibitor demonstrated that NO mediated its effect through the activation of guanylyl cyclase. Interestingly, 1 min after UVB irradiation, we observed a significant increase in cGMP content in melanocytes. The effects of UVB on cGMP production and on melanogenesis were blocked by both guanylate cyclase and NO synthase inhibitors. Additionally, inhibition of cGMP-dependent kinase also prevented the stimulation of melanogenesis by UVB and NO. Therefore, we concluded that NO and cGMP production is required for UVB-induced melanogenesis and that cGMP mediated its melanogenic effects mainly through the activation of cGMP-dependent kinase.Epidermal melanin is responsible for skin darkening and is synthesized by melanocytes as the result of a cascade of enzymatic reactions. Tyrosinase, which converts tyrosine to dopaquinone, is the rate-limiting enzyme involved in melanin synthesis and represents the major regulatory step in melanogenesis. Numerous stimuli are able to alter melanogenesis of cultured pigmented cells; vitamin D metabolites (1), retinoids (2, 3), melanocyte-stimulating hormone (4 -6), forskolin, cholera toxin, isobutylmethylxanthine (7,8), diacylglycerol analogs (9, 10), and UV irradiation (11)(12)(13)(14). Until now the molecular mechanism involved in the induction of melanogenesis by these agents was not fully elucidated. A role for the cAMP pathway has been proposed on the basis that adenylate cyclase activators such as melanocyte-stimulating hormone, forskolin, and cholera toxin or phosphodiesterase inhibitors (e.g. isobutylmethylxanthine) can stimulate melanogenesis (6). Additionally, the protein kinase C (PKC) 1 pathway was thought to be involved in the regulation of melanogenesis. Indeed, 1-oleyl-2-acetyl-glycerol, a PKC activator, stimulates melanogenesis (9, 10, 15), and a direct correlation between the level of PKC activity in melanocytes and the activity of tyrosinase has been shown (16). However, induction of melanogenesis by UVB or by 1-oleyl-2-acetyl-glycerol is unaffected by RO 31-8220, a PKC inhibitor (17), leading to the conclusion that protein kinase C does not play a pivotal role in the control of melanogenesis. In humans, only UVB radiation represents an established physiological stimulus of melanogenesis, and despite many efforts to identify the molecular events triggered by UVB radiation, the mechanisms underlying UVB-induced melanogene...