A male contraceptive must reduce the number of fertile sperm in the ejaculate to levels that reliably prevent fertilization (1). Conception can be prevented by diverting or suppressing sperm output and/or inhibiting sperm fertilizing capacity. So far, all male methods depend on mechanical means to reduce female exposure to sperm by traditional drug and device-free methods (abstinence, withdrawal, non-vaginal intercourse), condoms and vasectomy. No new male methods were introduced during the 20th century contrasting with numerous highly reliable, reversible contraceptive female methods developed over the last 4 decades using chemical (hormonal) methods. Unfortunately among existing male contraceptive options, the reversible methods are not reliable and the reliable method is not intentionally reversible. Despite these limitations, male involvement in family planning remains extensive. Globally, one third of couples using family planning rely upon contraceptive methods requiring active male participation (2) reflecting the traditional and ongoing reliance of family planning on male involvement. Greater participation by men in sharing the burdens as well as the benefits of effective family planning requires the development of more effective male methods. Despite strong community interest and medical agreement on the need for new, reversible, chemical male contraceptives as well as proof that reversible hormonal suppression of gonadal function is equally feasible for men as for women (3, 4), commercial development by the pharmaceutical industry continues to stagnate (5).