We investigated the effects of ingesting 75g of glucose on urinary excretion of electrolytes in patients with chronic glomerulonephritis (CGN) and diabetes mellitus (DM) over a 4-hour period. Creatinine clearance did not change in patients with either disease following glucose ingestion. Fractional excretion of sodium (Na), chloride (Cl) and potassium (K) decreased significantly, while that of calcium (Ca) and magnesium (Mg) increased significantly in both groups. The change in fractional excretion of phosphorus (P) differed between patients with CGN and DM. Fractional excretion of P increased initially, then decreased significantly in patients with CGN, while it decreased steadily in patients with DM. In diabetic patients, significant positive relationships were observed between cumulative filtered glucose and cumulative urinary excretion of Ca (r = 0.47, p < 0.05) and P (r = 0.54, p < 0.05). In addition, cumulative plasma IRI concentration was correlated inversely with cumulative urinary excretion of K (r = -0.54, p < 0.05) in diabetic patients. In nephritic patients. however, no significant relationship was observed among these variables. In conclusion, renal tubular reabsorption of Na, K and Cl was enhanced, while that of Ca and Mg was inhibited after glucose ingestion in both CGN and DM patients. Moreover, filtered glucose may be involved partially in the inhibitory effect on tubular reabsorption of Ca and P in diabetic patients.