The quantitative and qualitative effect of F.S.H. and F.S.H. in combination with thyroxine stress factors, elevation of temperature, length of photoperiod and relative humidy was studied. F.S.H. had increased the number and ratio of type B spermatogonia and had nor malized the decrease of the total number of spermato cytes in stress. The Sertoli ratio was higher than normal. F.S.H. had increased the total number and ratio of the spermatids of the treated group even higher than normal. Combination of thyroxine with F.S.H. had arrested the spermatogenic cell cycle. The tubules were lined only by spermatogonia. There was very prominant interstit ial cell hyperplasia.