The evidence that inhibin must exist.

The historical development of the 'inhibin' hypothesis is restated and the recent evidence reviewed. The circumstantial evidence for inhibin now seems very strong, and it probably acts in both sexes. Active materials have been isolated from several sources but it is not clear which is the circulating form of inhibin and a reciprocal relationship between circulating inhibin and FSH levels has not yet been demonstrated. Nevertheless, it appears that we may be optimistic about the existence of inhibin though the importance of its role as a physiological feedback inhibitor is still controversial.