Investigation of the regulation of testicular function has been enhanced by the ability to measure the dimeric, biologically active form of inhibin in men, inhibin B. This has allowed the demonstration that inhibin B is the afferent arm of the feedback loop from the testis that regulates FSH secretion, and investigation of inhibin B levels during reproductive development and in a variety of physiological and pathological states. Such studies have demonstrated many primary aspects of the relationship between FSH, testicular function (and in particular spermatogenesis) and inhibin B levels in blood, though many more specific questions remain. These include the precise nature of the relationship between inhibin B secretion and Sertoli cell function and how this relationship is influenced by the germ cell types present in the testis and by overall sperm production. When such information becomes available it will allow more accurate interpretation of blood concentrations of inhibin B. Similarly, apical secretion of inhibin B into seminal plasma may have considerable value in aiding assessment of the status of the seminiferous epithelium. Finally, neonatal secretion of inhibin B as a measure of Sertoli cell number and/or as a predictor of adult reproductive function offers novel possibilities for assessment and intervention.