The nipples of male tan guinea pigs were used to test the effects on the skin of oral contraceptive pills and drugs. The daily topically applied doses were 1/2,000 of a pill. After 3 weeks, the nipples treated with solutions of ethinyloestradiol-containing pills had grown (200–300%), the epidermis showed acanthosis (150–250%) and strong hyperpigmentation. Mestranol-containing pills as well as pure progestogen substances did not produce these effe