The search for a reversible male contraceptive has centered upon the suppression of sperm production or sperm motility. Gossypol, a natural substance extracted from the cotton plant, appears to cause both of these effects. Its ability to reduce spermatogenesis in men is undeniable and has been demonstrated in both large studies in China and a smaller confirmatory study in Brazil. These investiga- tions have revealed the remarkable fact that with gossypol, it is possible to separate an effect on the testis' gamete-producing function from an effect on its hormone-producing function. Thus, it is possible to maintain normal testosterone levels and libido while sperm counts (and motility) fall. Because of this unique and important action, gossypol warrants the fullest possible evaluation as a potential male contraceptive. Sheldon J. Segal vii ACKNOWLEDGMENT Lynn C. Landman played a major role in this publication by skillfully editing the manuscripts which were submitted for inclusion in this volume. Janet O'Connell added her efforts in editing, assembling texts and figures and handling final details required for publication. I thank these talented colleagues for their invaluable contributions.