In rats maintained for about three weeks on a diet inducing hyperlipidemia E-600 resistant acid esterase activities were markedly reduced in the aorta in comparison with untreated animals. In rats which were maintained on an ordinary diet containing the same amount of thiouracil as given to the hyperlipidemic animals, only a slight reduction of acid esterase activities was noted. Another group of animals was fed an ordinary diet for three weeks after three weeks on a hyperlipidemic diet. In the aorta of these animals the acid esterase activities were almost normal. There was little fat deposition in the aortas of animals given the various diets. No such effects on esterase activities were observed in the liver and lung of animals of the various groups. Inhibition of acid esterase activity was also observed in rats kept on a hyperlipidemic diet for 65 days. In these animals patchy deposition of partly anisotropic lipid was observed in the intima and media.