Liver damage was induced in adult male albino rats, weighting about 130-140 g, by oral administration of methomyl with single dose at two concentrations (1/10 or 1/20 LD₅₀/day for short term (4 weeks) and long term (8 weeks). The markers of liver damage were investigated by evaluating biochemical parameters in serum and liver tissues such as activities of aspartate aminotransferase (AST), alanine aminotrasferase (ALT)), alkaline phosphatase (ALP), total bilirubin (TB), total protein (TP), total lipids (TL), total cholesterol (TC), triacylglycerols (TAG) and malondialdhyde (MDA) levels. In addition serum urea, creatinine and uric acid were assessed after 4 and 8 weeks. The effect of oral administration. of grape seed oil (GSO) 4 ml/kg. Body weight/day on the above parameters for 4 and 8 weeks were investigated as a protective and antihepatotoxic effect. Oral administration of methomyl with single dose of concentration (1/10 or 1/20 LD₅₀/day for 4 and 8 weeks, showed significant increase (p<0.05) in serum liver enzymes activity (ALT, AST, ALP), TB, and MDA levels, while total protein showed a highly significant reduction as compared to negative control and control group received GSO, and the more effect was observed at the high dose of methomyl 1/10 LD₅₀. ALT was not affected at the methomyl dose 1/20 LD₅₀/day after 4 weeks. Serum lipid profile (TL, TC and TAG), creatinine and uric acid were also elevated in methomyl intoxicated rats with slight changes. in TC and TL, as compared to normal group at dose 1/20 LD₅₀/day after 4 and 8 weeks. A significant decrease (p<0.05) and a high decrease (p<0.01) in urea values were observed after short and long term in methomyl intoxicated rats. In liver tissues, elevation was found in ALP, TB, MDA, TC and TL levels after long term, while ALT, AST and total protein were significantly decreased, and Intoxicated group treated with methomyl 1/10 LD₅₀day for 4 and 8 weeks was more acute effect as compared to control groups. Oral treatment with grape seed oil GSO (4 ml/kg. body weight/day Improved all the above parameters being almost similar to control values with treated low dose of methomyl.