Current concepts of the biomembrane will be extrapolated to membranes of homeotherms to illustrate the influence of the nature of dietary lipid in nutritionally complete diets on membrane polar head group content and fatty acid composition. Utilizing animal models, the controlling influence of dietary long chain fatty acids on major lipid constituents of the mitochondrial membrane in cardiac tissue, the plasma membrane of liver, and the synaptosomal membrane in brain can be demonstrated. Diet-induced alterations in membrane composition arc associated with demonstrable changes in the function of specific membrane proteins. To illustrate this relationship, the effect of diet on mitochondrial ATPase activity and on a hormone receptor-stimulated function in the plasma membrane of the liver will be discussed. These observations suggest that the diet fat modulates enzyme functions in vivo by changing the surrounding lipid environment in the membrane.