To determine the effect of increased skin pigment on the cutaneous production of vitamin D3, circulating vitamin D concentrations were determined in two lightly pigmented Caucasian and three heavily pigmented Negro volunteers after exposure to a single standard dose of ultraviolet radiation (UVR). Exposure of Caucasian subjects to 1 minimal erythemal dose of UVR greatly increased serum vitamin-D concentrations by up to 60-fold 24-48 h after exposure, whereas this dose did not significantly change serum vitamin-D concentrations in Negro subjects. Re-exposure of one Negro subject to a dose of UVR six times larger than the standard dose increased circulating vitamin D to concentrations similar to those recorded in Caucasian subjects after exposure to the lower dose. These results indicate that increased skin pigment can greatly reduce the UVR-mediated synthesis of vitamin D.