It has been suggested that an increased erythrocyte Na-Li countertransport (Na-Li CNT) rate in patients with IDDM is associated to the risk of developing diabetic nephropathy. Little is known, however, about the possible influence of metabolic control on Na-Li activity. Aims of the study were to evaluate Na-Li CNT at the onset of IDDM and during the remission phase and its relationship with some clinical and metabolic parameters. Twelve insulin-dependent diabetic children (6 males, 6 females; mean age 10 +/- 0.6 years) were studied at the onset and 1, 4, 12 months after the diagnosis; 6 of them had a family history of hypertension. Twelve healthy children (6 males, 6 females; mean age 12 +/- 0.3 years) served as controls. As compared to control subjects (212 +/- 24 mumol/l RBC/h), red cell Na-Li countertransport activity of diabetic children was significantly higher at the onset (354 +/- 31 mumol/l RBC/h) of IDDM and at the first month (348 +/- 36 mumol/l RBC/h). Red cell Na-Li countertransport activity returned toward normal range at the fourth (239 +/- 33 mumol/l RBC/h) and twelfth month (162 +/- 34 mumol/l RBC/h). No correlation was found between the values of red cell Na-Li countertransport activity and those of clinical and biochemical parameters at any time. Patients with hypertensive relatives showed at baseline evaluation a significantly higher red cell Na-Li countertransport activity than those without (436 +/- 28 vs 273 +/- 34 mumol/l RBC/h; p < 0.002). This difference, although not statistically significant, was still evident at the late follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)