We have previously demonstrated a time-dependent variability in the diuretic effects of trichlormethiazide, a thiazide diuretic agent, in young rats. The study suggested that the time-dependent variations in urinary trichlormethiazide and susceptibility of renal tissues to the agent might be involved in this phenomenon. The present study was undertaken to test a hypothesis that such a daily variation in the effects of trichlormethiazide is blunted by age. Trichlormethiazide (0.5 and 2.0 mg/kg) was given orally at 1200 hrs (day trial) or at 2400 hrs (night trial) in young (10-11 week old) and aged (23-24 month old) Wistar rats. Urine was collected for 8 hours after the agent and urinary excretions of sodium, chloride and trichlormethiazide were determined. Urine volume and urinary excretions of sodium, chloride and trichlormethiazide following the agent were significantly greater at 1200 hrs than at 2400 hrs in the young rats. However these administration time-dependent changes in the effects of trichlormethiazide and its urinary amount diminished in the aged rats. In the day and night trials, there were significant correlations between urinary trichlormethiazide and its effects (urine volume, urinary sodium and chloride) in both groups of rats. The regression lines in each parameter of two trials differed in the young, but not in the aged group of rats. These data indicate that the mode of the time-dependent changes in the effects of trichlormethiazide is altered in aged Wistar rats. Dampening of the time-dependent variations in urinary trichlormethiazide and susceptibility to the agent might be involved in these chronopharmacological alterations in aged rats.