Photosynthesis of previtamin D₃ can occur throughout the epidermis and in the dermis when hypopigmented Caucasian skin is exposed to solar ultraviolet radiation. Once previtamin D₃ is formed in the skin, it undergoes a temperature-dependent thermal isomerization that takes at least 3 days to complete. The vitamin D-binding protein preferentially translocates the thermal product, vitamin D₃, into the circulation. These processes suggest a unique mechanism for the synthesis, storage, and slow, steady release of vitamin D₃ from the skin into the circulation.