Two trioleoyl glycerol hydrolases, one of lysosomal origin as determined by a high correlation with the lysosomal marker enzyme, N-acetyl-beta-glucosaminidase, and one having the characteristics of lipoprotein lipase, were measured at varying stages of lesion development in the aortas of cholesterol-fed rabbits. Both lipases were greatly enhanced in atheromatous aortas and were linearly related to lesion severity as measured by total aortic cholesterol. Lipoprotein lipase activities of myocardium and of plasma of cholesterol-fed rabbits were also significantly increased relative to controls. The data suggest that lipoprotein lipase might be a factor regulating cholesterol deposition in the aorta.