This paper reports the results of an examination of the timing relationships of the principal constituent components of the human electrocardiogram (ECG). ECG recordings were obtained from 21 healthy subjects, 10 male and 11 female aged between 13 and 65 years, over a wide range of heart rates extending from 46 to 184 beats per minute (bpm). A wavelet transform method based on the Mexican Hat wavelet was then used to precisely locate the positions of the onset, peak, termination and the duration of individual components in the ECG. Component times were then classified according to the heart rate associated with the cardiac cycle to which the component belonged. Second-order equations in the square root of the cardiac cycle time, TR-R of the form ATR-R1/2+BTR-R+C were fitted to the data obtained for each component to characterize its timing variation. These equations may be used to synthesize an ECG signal having a profile that varies with heart rate in a manner which reflects the in vivo variation.