The photosynthesis of previtamin D₃ from 7-dehydrocholesterol in human skin was determined after exposure to narrow-band radiation or simulated solar radiation. The optimum wavelengths for the production of previtamin D₃ were determined to be between 295 and 300 nanometers. When human skin was exposed to 295-nanometer radiation, up to 65 percent of the original 7-dehydrocholesterol content was converted to previtamin D₃. In comparison, when adjacent skin was exposed to simulated solar radiation, the maximum formation of previtamin D₃ was about 20 percent. Major differences in the formation of lumisterol₃ and tachysterol₃ from previtamin D₃ were also observed. It is concluded that the spectral character of natural sunlight has a profound effect on the photochemistry of 7-dehydrocholesterol in human skin.