Prostaglandins (PGs) are biologically active derivatives of 20 carbon atom polyunsaturated essential fatty acids released from cell membrane phospholipids. PGE2 and PGF2 are primary PGs. PGE2 is synthesized in skin and affects keratinocytes, Langerhans cells and melanocytes and regulates melanocyte proliferation. Epidermal melanocytes synthesize melanin in response to ultraviolet radiation (UVR). The mechanisms mediating the UVR-induced activation of melanogenesis are unknown but, as UVR induces the turnover of membrane phospholipids generating prostaglandins (PGs) and other products, it is possible that one of these might provide the activating signal. 1 In an in vitro study by Tomita et al .,2 normal human epidermal melanocytes became swollen and more dendritic when they were cultured with prostaglandin E 2 (PGE 2), but not with PGE 1. In another study by Nordlund et al ., 3 PGE2 applied topically to mice skin caused an increase in melanocyte density.