Our in vitro studies demonstrate that normal human epidermal melanocytes become swollen and more dendritic with an increase in amount of immunoreactive tyrosinase when they are cultured for several days with arachidonic acid metabolites, vitamin D3 or histamine. From these data we propose the following possible mechanisms for hyperpigmentations noted at postinflammatory sites and suntanned areas as well as at skin lesions of urticaria pigmentosa. Arachidonic acid metabolites and histamine, which are found in increased amounts in inflammatory skin, are thought to play a key role in the induction of postinflammatory hyperpigmentation. In sunburnt skin the increased proinflammatory mediators, particularly arachidonic acid metabolites, are also thought to stimulate melanocytes in the production of hyperpigmentation. Thus tanning after sun exposure may be induced not only by the effect of vitamin D3 and direct UV irradiation on the melanocytes but also by the effect of various arachidonic acid metabolites which are increased in sunburnt skin. Mast cells massively proliferate in the skin lesions of urticaria pigmentosa. Thus hyperpigmentation in the skin lesions of urticaria pigmentosa is quite likely to be induced by the chemical mediators, including histamine and leukotrienes, that are released from these cells.