The authors have previously reported the time-dependent change in the diuretic effects of furosemide, a loop diuretic agent, in young and middle-aged subjects. The current study was undertaken to examine an influence of aging on this chronopharmacologic phenomenon. Ten milligrams furosemide was given intravenously to 12 elderly subjects (greater than 70 years of age) at 9:00 AM (day trial) or at 9:00 PM (night trial) by a cross-over design. One-hour urine samples were collected for 3 hours after each administration, and urine volume and urinary excretions of sodium and furosemide were determined. Urine volume and urinary sodium excretion increased after furosemide administration. Contrary to the findings in the young and middle-aged subjects, no significant differences were observed in these parameters at any observation period between the day and night trials in the elderly subjects. Urinary furosemide excretion of the day and night trials did not significantly differ. These results suggest that the chronopharmacologic profiles of furosemide are altered in the elderly.