Quantitative measurements of the urinary excretion of gonadotrophins and androgen metabolites have been correlated with the functional condition of the germinal epithelium in various forms of male hypogonadism. Data on the influence of Leydig-cells, Sertoli-cells and the spermatogenetic stages on the gonadotrophin production are given. The finding of some, previously undescribed, lesions in spermatogenesis in certain clinical conditions is discussed. It is concluded that the testicular-hypophyseal feed-back mechanism in man depends on a very late stage in spermatogenesis. The suggestion is put forward that the cytoplasm which is split off from the spermatid immediately before liberation of the mature spermatozoon produces the hypophyseal inhibitor.