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The systolic time intervals have been measured in 76 normal children, aged 1 month to 15 years, from simultaneous recordings of the electrocardiogram, phonocardiogram and carotid arterial pulse or apexcar-diogram (left or right). Four different statistical methods were applied to study the separate influence of heart rate (HR) and age on these intervals. Left ventricular ejection time (LVET) had a highly significant correlation with HR but none with age. Age alone had a slight but significant influence on pre-ejection period (PEP) and isometric contraction time (ICT). Electromechanical systole (Q-II) varied directly with age and inversely with HR. Right and left total mechanical systole (TMSR, TMSL) and left isometric relaxa-tion time (IRTL) were inversely related to HR, while right electromechanical delay varied only with age. Q to first sound interval (Q-I), interval from onset of contraction to first sound on left and right apexcardiogram (CIL, CIR) and left electromechanical delay (EMDL) were found to be constant values. PEP/LVET (0.313, So 0.05) and TMSL/LVET (1.546, SD 0.128) were not significantly correlated with HR nor age. Regression equations and mean values are presented to permit rapid estimation of predicted normal values in children. The ratios TMSL/LVET and PEP/LVET being unaffected by age or HR, are suggested as practical indices of myocardial function in children