Systolic time intervals were measured in 253 normal children, with careful attention to precise recording and measuring techniques, to derive regression equations which could be used as reliable standards for clinical assessment of left ventricular performance in this age group. Using stepwise regression analysis, a highly significant correlation was found for electromechanical systole (QS2) and for left ventricular ejection time (LVET) with heart rate alone in both males and females, and age was not found to be a relevant variable. Similarly, for the preejection period (PEP) in males, heart rate was the only relevant variable. PEP in females showed a small but significant influence of age in addition to heart rate; in practice, this small influence can be neglected. Regression equations relating each of the systolic time intervals to heart rate alone were therefore derived for clinical use. The PEP/LVET ratio was found to be only weakly correlated with heart rate in males and with age in females, and in practice can be considered to be independent of these variables. The mean value of the PEP/LVET ratio and its standard deviation (0.30 +/- 0.04) can therefore be used for all children. We found that satisfactory recording of systolic time intervals can be obtained with little difficulty in almost all children using the techniques described. The method is therefore practical for clinical use in children, as it is in adults, and the normal standards derived in this study can serve as a reliable basis for its application.