Investigations of the influence of dietary protein level on the activity of hepatic microsomal epoxidase were undertaken to explain the effects of low protein diets on heptachlor toxicity. These studies showed that low protein diets resulted in lower levels of microsomal epoxidase when compared to controls fed adequate protein. The in vitro determination of epoxidase correlates closely with the observation that low protein diets cause a reduction in toxicity of hepta chlor to weanling rats. J. Nutr. 101: 9-12, 1971.