The influence of age on the nocturnal fall in blood pressure (BP) was examined in essential hypertensive patients as well as normal subjects. BP was monitored every 5 min for 24 hr by means of a finger volume oscillometric device. Average daytime BP was similar in the 3 age groups [young: less than 40 (years), n = 49, average daytime systolic BP (ASBP) = 132 +/- 20 mmHg, average daytime diastolic BP (ADBP) = 82 +/- 17 mmHg; adult: 40 less than or equal to less than 60, n = 110, ASBP = 127 +/- 19 mmHg, ADBP = 86 +/- 13 mmHg; old: 60 less than or equal to, n = 33, ASBP = 131 +/- 17 mmHg. ADBP = 83 +/- 11, mean +/- S.D.]. The nocturnal fall in BP was observed in all age groups and its amplitude (delta BP = average daytime BP - average nighttime BP) in the old patients (delta SBP = 13 +/- 11 mmHg, delta DBP = 10 + 8 mmHg) was similar to that in the young patients (delta SBP = 11 +/- 8 mmHg, delta DBP = 10 +/- 8 mmHg). The results suggests that information on the nocturnal behavior of BP is valuable in treating aged essential hypertensives to prevent cerebral and/or myocardial ischemia during sleep.