Urinary albumin excretion during exercise was measured with a radioimmunological method in a group of 13 young male diabetic patients and in a comparable control group. The duration of diabetes was 2-18 years; they had no proteinuria (Albustix¿) and no other signs of renal disease. There was no difference in the basal albumin excretion. In the diabetics the average albumin excretion was doubled during exercise at 600 kpm/min for 20 min, from 9.1 mug/min to 18.7 mug/min (P less than 0.005). No significant change was seen in the controls. These results strongly suggest that abnormal glomerular filter properties are present in patients with relatively short duration of diabetes - that is, in patients who are known to have thickened glomerular basement membrane. The exercise provocation test may be useful in other fields of renal pathophysiology.