In order to help clarify the effects of hyperthyroidism on renal function and electrolyte metabolism, we measured the venous plasma concentrations of urea, creatinine, urate, hydrogen ion and electrolytes, and the urinary concentrations of total protein, albumin, retinol-binding protein, N-acetyl-beta-D-glucosaminidase activity, and creatinine in patients when hyperthyroid and again after they had been euthyroid for at least 4 months. Significant (P less than 0.05) decreases in the mean plasma concentrations of urate and chloride and significant increases in creatinine, total CO2 and hydrogen ion mean concentrations were observed when the patients became euthyroid. The mean concentrations of sodium, potassium and urea did not change significantly. The values of the ratios total protein/creatinine, albumin/creatinine, N-acetylglucosaminidase/creatinine and retinol-binding protein/creatinine were all significantly (P less than 0.05) elevated in random urine specimens obtained from hyperthyroid patients as compared to the values when euthyroid. Mild proteinuria occurs in most thyrotoxic patients which does not appear to be due predominantly to either glomerular or tubular renal injury. The changes in plasma analytes that were observed may be attributed to increases in glomerular filtration rate and tissue nucleic acid turnover and a tendency to respiratory alkalosis in the hyperthyroid patients.