To delineate the effects of true aging, undetected heart disease and deconditioning on heart rate, we performed 24-hour ambulatory electrocardiography and maximal exercise stress test on 101 subjects with normal hearts. The maximal heart rate recorded was 180 beats/min; the minimum was 35 beats/min. A distinct diurnal pattern was observed. With increasing age, a decrease of the maximal heart rate achieved during exercise stress test (r = 0.27, p = 0.05) or spontaneously recorded during the day (r = 0.41, p = 0.0005) or night (r = 0.24, p = 0.03) was observed. The resting and average heart rates were not affected by age. Older subjects had lower exercise tolerance (r = 0.41, p = 0.0001). Low exercise tolerance was associated with higher increments of heart rate for submaximal exercise levels (r = 0.0001) and lower maximal heart rates (r = 0.43, p - 0.008). These changes of heart rate with age are not due to undetected cardiac disease, because the subjects included in the study were meticulously screened by noninvasive and invasive means.