The effects of methylparathion (MPT) on reproductive system of the male frog Rana rugulosa were investigated. Male frog ages of 2, 4, 6, 8, 10 and 12 months old were exposed to 20 ppm of MPT for 60 days. At the end of the experiment, animals were autopsied. Plasma samples were collected and kept at -20 ํC until testosterone (T) radioimmunoassay. Testes were fixed in Bouin's solution for histological study. MPT significantly lowered plasma T levels of the frog at all ages examined except of the frog at the age of 2 months. Plasma T levels decreased 15.94% in 4 month old, 17.68% in 6 month old, 29.05% in 8 month old, 19.18% in 10 month old and 27.12% in 12 month old frogs. Microscopic studied showed that MPT directly injuried Leydig cell and effected on cell structure. The histological alteration of Leydig cell composed of hydropic swelling, pyknotic nucleus, perinuclearchromatin clumping nucleus, karyolysis, nuclear membrane disruption and vacuolated of interstitial tissue. It is therefore concluded that the effect of MPT on lowering of T levels may exert directly on the Leydig cells.