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The present study was undertaken to examine the effect of age on diurnal variation in glomerular filtration rate (GFR). The GFR, as estimated by creatinine clearance (Clcr), was determined during a 24-hour period in 10 younger (mean ± SD age 42 ± 9 years) and 10 older (mean age 75 ± 4 years) patients with hypertension. Significant diurnal variations in Clcr were observed in the younger patients, with a peak during the day and trough during the night. Such were not observed in the older patients, however. These results suggest that diurnal variation in GFR is affected by age. Chronopharmacologic profiles of drugs, which are mainly excreted in urine by glomerular filtration, might be altered in these patients.