Although heart rate dependency of QT interval is well known, the relationship of other electrocardiogram (ECG) parameters to heart rate has been researched less intensively. This study investigated the heart rate dependencies of QT interval, PQ interval, and QRS width in 40 healthy subjects (18 women; mean age, 30.4 +/- 8.1 years). In each subject, 3 long-term (approximately 13 hours) 12-lead ECGs were obtained for 3 day-time periods with gaps of 2 to 3 weeks between repeated recordings. In each recording, approximately 230 ECG measurements of QT interval, PQ interval, and QRS width were made, each preceded by stable heart rate. For each recording, linear regression slopes of QT/RR, PQ/RR, and QRS/RR relationships were obtained. Intrasubject SDs of individual values were compared with intersubject SDs of intrasubject means to test the individuality of the relationships. The intrasubject means of the slope values were also compared between sexes. The individual SDs of the QT/RR regression slopes were 0.0116 +/- 0.0065, whereas the population SD of intrasubject means was 0.0245 (P = 3.6 x 10(-15)). For the PQ/RR slopes and QRS/RR slopes, these values were 0.0085 +/- 0.0050 vs 0.0314 (P = 7.9 x 10(-28)) and 0.00189 +/- 0.00157 vs 0.00550 (P = 2.6 x 10(-17)), respectively. The QT/RR slopes were steeper in women than in men (0.194 +/- 0.019 vs 0.168 +/- 0.022, 0.0005), whereas the QRS/RR slopes were, on average, negative in women while positive in men (-0.00138 +/- 0.0045 vs 0.00335 +/- 0.0054, P = .005). There were no sex differences in the PQ/RR slopes (0.054 +/- 0.032 in women vs 0.055 +/- 0.031 in men, P = .95). Thus, not only the heart rate dependency of QT interval but also the rate dependencies of PQ interval and of QRS width show high intrasubject stability with substantial intersubject differences.