One clinical complication of immunosuppressive cyclosporine A (CsA) therapy is the stimulation of hair growth. Since few pharmacologic agents cause hypertrichosis, CsA appears particularly interesting for investigating the mechanisms which control normal hair formation. Previous investigators have shown that CsA affects the hair growth of the laboratory rat, several genetic variants of mice, as well as humans, and they have concluded that CsA influences keratinization predominantly. Using a well-defined in vivo assay which measures the induction of hair follicle growth, we report here that CsA induces resting (telogen) follicles to enter active growth (anagen) in normal laboratory mice (C57 B1-6), i.e., animals with a normal hair cycle. The experiments indicate that the rate of anagen induction is dependent on the dose, time course, and method of administration and that it may be mediated via a direct action of CsA on the skin and its appendages. These studies suggest that understanding the molecular mechanisms of CsA action on hair growth will help elucidate the mechanisms of normal anagen induction.