The pathologic changes in the aorta, liver, and adrenal glands, and the changes in sterols of plasma and liver induced in birds by the prolonged feeding of a diet containing 0.5 per cent Δ4-cholestenone are described. Lesions were found in the media of thoracic aortas and intima of the abdominal aortas of the sterol-fed birds and evidence is presented to show that the crystalline material in the abdominal plaques is not cholesterol. There was enlargement of both liver and adrenal glands with a pronounced deposition of total digitonin-precipitable sterols which could not be accounted for by Liebermann-Burchard reacting sterols (LBRS). In the Δ4-cholestenone-fed birds the level of LBRS in plasma, fell to about 50 per cent of that in control birds, whereas the levels of total digitonin-precipitable sterols (TDPS) rose to values of 200 mg. per 100 ml. The fraction of the TDPS present as non-LBRS was quite constant in the plasma of the sterol-fed birds (63–69 per cent). By means of isotope dilution procedures the non-LBRS in the livers of sterol-fed birds was identified as dihydrocholesterol.