Six normal dogs were made galactosemic by feeding a 30% D-galactose diet, and were followed up to 5 yr. For comparison, 10 normal dogs and 10 alloxan-diabetic dogs were concurrently fed the diet less the galactose supplement. Retinopathy occurred in each of four dogs glactosemic 3 or more yr, and was absent at lesser durations of galactosemia, and from normal dogs not given the galactose supplement. The retinopathy was marked by saccular capillary aneurysms, hemorrhages, nonperfused or acellular vessels, tortuous hypertrophic capillaries, loss of capillary pericytes, and other lesions typical of diabetic patients and alloxan-diabetic dogs. In galactose-fed dogs, blood galactose varied between 0 (fasted) and 250 mg/ dl (postprandial), and gly cosy la ted hemoglobin levels became supranormal. In contrast to diabetic dogs, blood levels of glucose, free fatty acids, and branchedchain amino acids were not elevated in the galactosemic dogs, and their serum insulin seemed normal. The results suggest that the level of blood hexose is itself an important determinant of retinopathy.