Several studies on the adverse effects of methotrexate have been reported, especially its implication in the degeneration of spermatogenesis, reduced sperm count and ultimate male infertility. As an antagonist, methotrexate (MTX) uses folic acid to obstruct the production of some biomolecules involved in synthesis of DNA, RNA and protein. It is used in the treatment of cancer and other diseases such as psoriasis, and rheumatism. Reports have also revealed that the expression of Bone Morphogenetic Protein (BMP8a) promotes spermatogenesis and fertility through the induction and activation of signaling sets of transcription factors, SMAD1/5/8. Hence, the expressions of these proteins and role of apoptosis are crucial to understand the mechanism involved in Methotrexate-induced infertility. In view of this, albino mice (Swiss strain) were randomly sorted to four groups. Group I served as control while groups II, III & IV (n=5) were treated with 5, 10 and 20mg/kg/day of Methotrexate (IP) respectively. Expressions of BMP8A and SMADs 1/ 5/ 8 were done by PCR and Western blotting techniques. Germ cell apoptosis was measured by flow cytometry and immunohistochemistry techniques. Ultrastructural changes were assessed in leydig cells as well as sertoli cells. The results of this study reveal a down-regulation of BMP8A and SMAD1/5/8 proteins in a dose-dependent pattern. Induction of apoptosis was also confirmed by the expression of primary apoptotic Bax antibody. The sertoli cells which play their major roles of nourishing and protecting the development of sperm cells were severely impaired too. These findings suggest that the function of BMP8A and SMAD1/5/8 proteins in promoting proliferation and differentiation of spermatogonia was severely disrupted following methotrexate exposure. Caution should therefore be taken when administering this drug.