Effects were studied of insulin and of varying the concentration of glucose upon L-leucine-14C (UL) metabolism by isolated rat epididymal adipose tissue cells. At lower glucose concentrations, insulin increased CO2 formation and lipid synthesis from labelled L-leucine. At higher glucose concentrations insulin decreased apparent CO2 formation and lipid synthesis. Insulin stimulated protein synthesis at all levels of glucose concentration studied, but the increment was less at higher glucose concentrations.