To assist the attending physician who may have difficulties in deciding whether observed deviations from normal renal physiology are disease-induced or diuresis-induced we studied urinary excretion rate of proteins and protein degradation products in eight healthy male volunteers between 60 and 70 years. After a 24 hr control period in the ward the subjects received 80 mg furosemide intravenously. The 24 hr creatinine clearance was reduced from 95 +/- 9 to 83 +/- 6 ml/min. (P less than 0.05). The reduction in the 24 hr carbamide clearance from 35.8 +/- 9.4 to 33.1 +/- 6.2 ml/min. was not significant. The urinary pH was significantly increased during the first hour but after the pH showed a fall which was significant during 2-8 hours after the injection. Initially the urate excretion showed an apparent increase but from 1-8 hours after the injection significantly less urate was excreted. The 24 hr urate clearance was reduced by furosemide from 5.3 +/- 1.6 to 3.4 +/- 1.1 ml/min. (P less than 0.05) and the amount of urate excreted was reduced from 2.6 to 1.6 mmol per 24 hrs (p = 0.05). The excretion rate of beta 2-microglobulin was reduced (P less than 0.01) whereas the excretion rate of albumin was increased (P less than 0.05).