Studies were performed in 22 patients with diabetes mellitus type I, in 19 patients with diabetes type II, and in 15 healthy subjects. In all subjects the following parameters were assessed: creatinine clearance and 24 h urinary excretion of THP, albumin, sodium, potassium, and calcium. All studies were performed twice: first after 3 days on a normal sodium diet and subsequently after 3 days of sodium restriction (30-40 mmol Na/24 h). Urinary excretion of sodium was significantly lower in patients with diabetes mellitus type II than in healthy subjects. In patients with diabetes mellitus type I urinary excretion of sodium was significantly lower than in controls only after sodium restriction. No significant correlation was found between urinary excretion of THP and electrolytes. Results obtained in this study do not confirm presence of any relationship between urinary excretion THP and electrolytes.