The desired biologic effect of progestins used in OCs is progestational activity. Undesired pharmacologic properties such as androgenic activity are not necessary for contraception and increase the potential for adverse effects. A selective progestin has progestational effects at relatively low concentrations or doses and androgenic effects at only relatively high concentrations or doses. The degree to which progestational activity is maximized and androgenic activity is minimized is a measure of a progestin's selectivity. The ratio of its affinity for progesterone receptors to its affinity for androgen receptors is the selectivity index. To minimize the androgenic side effects associated with the older progestins, the doses used in OCs have been reduced over the years. These dose reductions have decreased the potential for undesired androgenic effects but also have negatively affected cycle control. Three new progestins, norgestimate, desogestrel, and gestodene, have relatively greater affinity for progesterone receptors than for androgen receptors when compared with the older agents, permitting a reduction in androgenic side effects without the need for further dose reduction. Preclinical receptor-binding studies and animal pharmacologic studies have documented the higher selectivity indexes of these new progestins. Their higher ratios of progestational to androgenic activity provide the basis for the reduction in androgenic adverse effects observed with their clinical use.