Neutralizing antisera to rat follicle stimulating hormone (FSH) and sheep luteinizing hormone (LH) have been prepared and made monospecific by suitable absorption. The antiserum to rat FSH is devoid of contaminating antibodies to LH, binds only to I125 FSH, and neutralizes the biological activity of rat FSH in the HCG augmentation assay. The anti-sheep LH serum cross-reacts with rat LH and causes abortion when injected into 8-day pregnant rats. Administration of anti-LH serum to 20 day male rats, over a 14 day period, resulted in an 80% reduction in testes weight, accompanied by a marked reduction in the weights of the epididymides, ventral prostate and seminal vesicles. The size of the Leydig cells, the diameter of the seminiferous tubules, the number of germ cells and serum testosterone levels were all dramatically reduced. Treatment with anti-FSH serum decreased the weight of the testes by 50% without any reduction in the weights of the accessory organs or in the levels of serum testosterone. The Leydig cells appeared normal; however, the number of spermatids and spermatocytes was much reduced when compared to controls. Thus, it appears that selective withdrawal of FSH interferes with the initiation of spermatogenesis, without alterations in the serum testosterone levels. Attempts are currently underway to determine whether FSH is required for normal spermatogenesis in adult rats. The structure and function of the Sertoli cell will be carefully examined in all our FSH treated rats. BIBLIOGRAPHIC REFERENCES: Dym, M. and H.G. Madhwa Raj (1975) Early responses of the testis to anti LH serum. Proc. of the Amer. Assoc. of Anat. Anat. Rec. 181: 350. Raj, H.G. and M. Dym (1976) The effects of selective withdrawal of FSH or LH on spermatogenesis in the immature rat. Biol. Reprod. In Press.