Background:Age and gender are both known to affect heart rate(HR) dynamics. Improved time-domain measures and new nonlinear methods based on fractals and chaos theory may uncover subtle abnormalities that are not detected with the conventional measures. Objective:The aims of this study is to (1) systematically characterize and quantify the effects of aging and sex over a broad age range on HR dynamics, and (2) compare the conventional time- and frequency-domain measures with nonlinear measures. Methods and Results:The effect of age and gender on HR dynamics studied in 1,049 healthy adults(559 men, age range 20 to 83 years), divided into 6 age strata based on 10-yr interval. HR variability of ten minutes ECG recording during controlled environment was used for analysis of the following parameters: Frequency- and time-domain measures including pNNx with variable thresholds (from 10 to 100 ms), short-term (慣1) and intermediate-term (慣2) fractal scaling exponents, approximate entropy (ApEn), sample entropy (SampEn), corrected Shannon entropy (CSE), and the power-law slope (棺) of the R-R interval time series. Age-dependent progressive decrease of NN20(r=-0.501, P<10^-66) and PNN20 ((r=-0.419, P<10^-45) were observed, which was greater than that of standard PNN50(r=-0.337, P<10^-29). Loss of complexity( decreased ApEn, (r=-0.104 P=0.001) and altered fractal-like heart rate behavior(decreased 慣1, (r=-0.192, P<10^-9 and increased 慣2, (r=0.316, P<10^-25) were observed with aging, but significant change between age groups is evident after the fifties. These results were the same after adjustment of clinical factors affecting HR dynamics, such as sex, body mass index, blood pressure, fasting blood sugar and lipid profile. Conclusions:Loss of complexity and altered fractal-like heart rate behavior is observed with aging. Enhanced discrimination of age-related change of R-R interval dynamics is obtainable with pNNx parameters using lower threshold of 20 ms than the standard 50 ms.