Minoxidil, a potent antihypertensive agent, has the unique side effect of stimulating hair growth and it has been suggested that minoxidil sulfate may be the active form of minoxidil. In this study, we investigated whether sulfation of minoxidil in various cells of the skin; hair follicle, proliferating and differentiating keratinocyte and fibroblast occurs. We provide evidence that sulfation of minoxidil occurs in the hair follicles and proliferative keratinocytes. On the other hand the activity in differentiating keratinocytes and fibroblasts was extremely low. The strong sulfation of minoxidil that occurs in hair follicle cells may be related to the hair growth stimulating effect of this drug.