The sodium and potassium concentrations of the red blood cells and the serum were investigated in 21 children with diabetes mellitus. Measurements were made prior to and 1 and 2 hours following insulin administration. Before insulin treatment, the sodium level in the red blood cells of the diabetes patients was significantly higher that the control level, while the potassium level was significantly depressed. These differences were not observed 1 and 2 hours following insulin administration. Attention is drawn to the rapid and extensive electrolyte changes in the red blood cells of diabetics in response to insulin. The potassium content of the serum of the diabetics was significantly decreased by insulin administration. No correlation was found between the electrolyte levels and the fasting blood glucose levels. A weak negative correlation (p less than 0.02) was observed between the potassium content of the red blood cells and the duration of the illness. The pathogenetic, diagnostic and therapeutic implications of the results are discussed.