It has not been determined if signal-averaged electrocardiograms (SAECGs) have a diurnal variation. We recorded 3-channel 24-hour ECGs in 30 healthy volunteers and calculated the following parameters for the first 500 sec of every hour over the 24-hour period: the mean heart rate, the filtered QRS duration (f-QRS), the root-mean-square voltage of the signals in the last 40 msec of the QRS (RMS40), and the duration of low-amplitude signals (< 40 microV) in the terminal portion of the QRS (LAS40). Heart rate and RMS40 increased in the daytime compared with the nighttime; f-QRS and LAS40 increased during the nighttime hours. The f-QRS and LAS40 were negatively correlated with heart rate in all and 22 (73%) of 30 subjects, respectively, and RMS40 was positively correlated with heart rate in 21 subjects (70%). In conclusion, SAECGs exhibit a clear diurnal variation in healthy subjects, which is closely related to changes in heart rate.