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The principal aim of the present study was to evaluate median lethal dose (LD50) of fenvalerate, a synthetic pyrethroid (Type II) insecticide, in male Wistar rats. Median lethal dose (LD50) is defined as the dose which is lethal and proved to cause death in half of the experimental group of animals after specified test duration. The LD50 is a standardized measure for expressing and comparing the toxicity of chemicals. This test is proved to be an initial screening phase for the evaluation, measurement and comparing of the acute toxicity of a chemical. Fenvalerate dissolved in groundnut oil was administered to 5 different groups of animals (10 animals in each group) as a single dose by oral intubation. Based on the pilot study the selected doses of fenvalerate were 400,425,450,475 and 500mg/kg b.wt. for male rats. The animals were kept under observation for any type of toxic symptoms and death in 96 hours. The percentage of dead animals after 96 hours was calculated, which was then transformed into probits for the estimation of median lethal dose (LD50). In this study, the calculated median lethal dose (LD50) of fenvalerate dissolved in groundnut oil was found to be 434.51±29.90 mg/kg b.wt. in Wistar rats. In sub-chronic and chronic studies, this data is very helpful for the establishment pf the dosage regimen.