Diabetic rats were treated with insulin at various dosages and for various periods of time. The influence of metabolic control on the synthesis of protein and basement membrane collagen of isolated glomeruli was investigated. The protein and basement membrane collagen synthesis was increased in the untreated diabetic rats compared to non-diabetic controls. The synthesis was not affected by high doses of insulin with brief normalization of the blood sugar. Insulin treatment from the beginning of diabetes only led to a normalization of protein synthesis in moderate metabolic control. On the other hand, a rise of the basement membrane collagen synthesis could only be prevented by a strict metabolic control of the rats. The results show that basement membrane collagen synthesis reacts very sensitively to the diabetic metabolic situation. Insulin deficiency itself does not appear to be alone responsible for alterations of basement membrane synthesis in diabetes.