When cyclosporin A (CyA) at doses of 10 and 100 mg/kg body weight/day was given for 14 days to six-week-old female and male nude mice, the length of the hair of CyA-treated mice began to increase from day 7 of the treatment at both doses. There was no hair growth in the control mice, which were given only olive oil. The hair length of CyA-treated mice was 3 to 4 times longer than that of control mice. CyA at both doses induced hair growth to the same extent. There was no difference in the hair growth between the male and female mice. The growth of hair was apparent on the neck, back, and hip. However, no hair growth was observed in the abdominal region. The hair length of the CyA-treated mice decreased after cessation of the treatment, and returned to the level of the control mice on day 14 after the end of the treatment. Microscopic examination revealed that there was no difference in the number of hair follicles between the control and the CyA treated mice.