Insulin receptors of liver and brain plasma membranes were investigated in rats kept on standard (S), high-carbohydrate (HC) and high-protein (HP) diets. Both liver and brain show similar tendency to reduce number of high affinity receptors and their affinity to the hormone in rats fed a HP diet. After a HC diet the effects seem to be opposite. Animals show nearly the same number of receptors as in the control group but the affinity of insulin binding to liver -but not brain plasma membranes is enhanced.