The hypothesis that total body exchangeable sodium (ENa) is elevated in type 1 (insulin-dependent) diabetic patients with short-duration diabetes and no signs of microangiopathy was tested. Also tested was whether peripheral hyperinsulinaemia, in terms of the amounts of insulin injected subcutaneously, contributes to the increased ENa. Three studies were performed. Study 1 was a cross-sectional study comprising 28 type 1 diabetic men (aged 18-35 years) with short-duration diabetes (< 5 years) and no signs of diabetic complications, and 22 control subjects. Study 2 was a prospective study of 17 newly diagnosed diabetic patients (aged 20-35 years, median 27 years) who were studied on two occasions on different insulin doses. Study 3 was a 12-month prospective intervention study of 21 type 1 diabetic patients with incipient nephropathy, who had been randomized either to receive continuous subcutaneous insulin infusion for improvement of glycaemic control or to remain on conventional insulin treatment. In study 1, ENa was higher in short-duration type 1 diabetic men than in controls (3003 +/- 325 vs 2849 +/- 207 mEq/1.73 m2, P < 0.05) and was correlated significantly with the insulin dose (r = 0.38, P < 0.05). In study 2, of the newly diagnosed diabetic patients, 11 received a reduced insulin dose and 6 an increased dose as compared with the initial study. ENa was reduced in all patients receiving less insulin (P < 0.001) and remained unchanged in patients receiving more insulin.(ABSTRACT TRUNCATED AT 250 WORDS)