Liver is removed from albino rat and assayed for total adenosine triphosphatase (ATPase) activity following administration with single and repeated doses of larvin and cypermethrin. The data indicate that the total (Na+, K+; Mg2+) dependent ATPase in the liver tissue is significantly inhibited by single and repeated doses of both insecticides. This inhibition is more pronounced by the repeated dose of cypermethrin than that of larvin. The in vitro study revealed that the inhibition encountered by different concentration of both larvin and cypermethrin is of the irreversible non-competitive type. This data indicate that these insecticides can cause biochemical and histopathological changes in the liver ATPase activity which may inhibit several biochemical functions of ATPase system such as: the active transport of metal ions, oxidative phosphorylation of liver cells and generally the muscle contraction.