When human skin was exposed to simulated solar ultraviolet radiation, epidermal 7-dehydrocholesterol was converted to previtamin D₃. During prolonged exposure to simulated solar ultraviolet radiation, the synthesis of previtamin D₃ reached a plateau at about 10 to 15 percent of the original 7-dehydrocholesterol content, and previtamin D₃ was photoisomerized to two biologically inert isomers, lumisterol₃ and tachysterol₃. Increases either in skin melanin concentration or in latitude necessitated increases in the exposure time to simulated solar ultraviolet radiation required to maximize the formation, but not the total content, of previtamin D₃. In order of importance, the significant determinants limiting the cutaneous production of previtamin D₃ are (i) photochemical regulation, (ii) pigmentation, and (iii) latitude.