Methyl parathion (MP: O,O-dimethyl-O-4-nitrophenyl phosphorothioate) is an organophosphate pesticide used in agriculture to protect a variety of crops. Food stuffs such as fruits and vegetables could be contaminated with MP, which may be a potential route of exposure. Previous studies have shown that MP is a reproductive toxicant in animal models. The present study was designed to investigate the mechanism of symplast formation and biochemical changes that occur in the testis, following MP exposure. MP was treated to adult male Wistar rats (N=5/dose/sample time) as follows. Experiment 1 - 0, 0.75 or 1.5mg/kg/d i.p. for 25 days and experiment 2 - 0 or 3.5mg/kg/d p.o. for 25 days and sacrificed on Day 17, after the last exposure. Light microscopic examination of testis was made to evaluate the structural changes and also to establish a process of symplast formation and destruction. Quantitative biochemical estimations were made in the testis for acid phosphatase (ACP), cholesterol, total protein, uric acid, and lactate dehydrogenase (LDH). MP induced structural changes in the testis in consensus with the previous studies. The symplasts were found in the testes in experiment 1. Those cells were formed due to the cell fusion of round spermatids. The symplasts were degenerated by nuclear fragmentation. The nuclear fragments were extruded from the symplasts leaving behind only the eosinophilic cytoplasm. The cell fusion and multinucleated giant cell formation was the reason for MP induced tubular atrophy. Number of tubules with symplasts increased in experiment 1 in a dose-dependent pattern. Johnsen's scores also decreased in a dose-dependent manner in experiment 1 indicating a dose-dependent tubular destruction. The ACP, cholesterol, total protein, and LDH levels decreased in both experiments against their respective controls, whereas the uric acid level decreased in experiment 1 and increased in experiment 2 (P<0.01-0.001). The effects in experiment 1 were dose-dependent. In conclusion, MP induces the formation of symplasts by cell fusion of round spermatids, which is a process involved in tubular atrophy and also induces biochemical changes in the testis.