No significant changes of LH in basal conditions and in response to GnRH stimulation were found in any of the subjects studied; the same applies to basal testosterone and testosterone after HCG stimulation. On the other hand, the pituitary response for FSH gonadotropin showed significant variations in the different groups of subjects examined. These variations were closely correlated to the stages at which spermatogenesis was arrested. The earlier the stage of spermatogenetic arrest, the more strongly enhanced the FSH response to GnRH stimulation. Further, the most significant change, and the one most liable to be suited for purposes of diagnosis, was the one observed when passing from the 1st to the 2nd degree arrest (spermatids-spermatocytes). This FSH pattern is a constant finding; so much so that in the view of some authors testicular biopsy can now be avoided in cases with strongly increased pituitary FSH reserve since in these conditions the seminal line has been shown not to go beyond the stage of the spermatocyte.