The relationship between urinary protein excretion and control of diabetes was evaluated in alloxan-diabetic dogs prospectively assigned to poor, moderate, or good glycaemic control. Protein excretion rate increased with the duration of insulin deficiency, and was significantly greater than normal in the poor control group by the fourth year of diabetes. Appreciable differences in the severity of the proteinuria were observed among animals of the poor and moderate glycaemic control groups; some of the animals excreted in excess of 500 mg protein/24 h while others excreted no more than normal throughout the 5 years of study. Differences in glycaemic control among these insulin-deficient animals seem not sufficient to account for the observed differences in protein excretion. Immunoassay for albumin indicated that the defect resulting in supranormal protein excretion was at least partly glomerular in origin. Good glycaemic control prevented the protein loss from exceeding normal. A potential role of hyperglycaemia in the development of proteinuria was examined in nondiabetic dogs made experimentally hyperglycaemic with galactose. Consumption of a 30% galactose diet for up to 5 years duration had little influence on protein excretion.