We investigated the ultrastructural effects of methamidophos and the positive effects of 2-pralidoxime (2-PAM) on the liver. Male Wistar-albino rats were assigned to 4 groups and all were treated per os: Group I (n=10) received 30 mg/kg methamidophos; Group 2 (n=7) (serving as controls for Group 1) received physiologic NaCl; Group 3 (n=10) received 30 mg/kg methamidophos and was treated with 2-PAM and atropine when cholinergic symptoms were noted; and Group 4 (n=7) (serving as controls for Group 3) was treated with physiologic NaCl. Plasma cholinesterase was measured using radioimmunoassay. Liver tissues were prepared for electron microscopic studies. Methamidophos treatment of Group 1 led to serious changes in hepatocytes and organelles. These changes were not detected in Group 3. In Group 1, the chromatin content of some hepatocyte nuclei and cytoplasmic density increased; these cells also became vacuolar in appearance as a result of lysis in the mitochondrial matrices. In some cells, the lipid content constituted the majority of the cytoplasm. Furthermore, these cells were surrounded by glycogen accumulation. In some areas of the perisinusoidal zone, collagen fibers had increased to form bands. None of these changes were noted in Group 3. These findings suggest that acute organophosphate poisoning causes serious histopathological effects in rat liver, but that these changes are reversible with appropriate treatment strategies.