Both dimethyl methylphosphonate (DMMP) and trimethylphosphate (TMP) are organophosphorous compounds that can evoke sterility in male rodents. The following studies examined the pathology of reproductive organ, especially on the testis, by light microscopy after treatment with both agents. Adult male rats were treated per oral with DMMP, 1,750 mg/Kg, for up to 12 weeks and per oral with TMP, 400 mg/Kg for up to 5 weeks. After 5 weeks of treatment with DMMP there were occasional multinucleated giant cells composed of late spermatids in stages X, XI, XII as well as cytoplasmic vacuolation of Sertoli cell. Anachronistic spermiations were seldom, if ever, seen throughout the experiment. After 7 weeks of DMMP those were markedly diminished. The overall changes after treatment with TMP are somewhat similar to those treated with DMMP. The major changes were composed of aggregate of multinucleated giant cells and maturation arrest at spermatid level, which appear immediately after administration of TMP. The peak frequency in the emergence of multinucleated giant cells in treatment with TMP was noted just a week after treatment, but afterwards declined. Maturation arrest was prominent after 3 weeks in the cases treated with TMP.