The study was designed to evaluate inter-institutional variability in systolic time interval (STI) measurement. STI records were obtained from 8 institutions in Czechoslovakia. Altogether 262 normal subjects of both sexes were evaluated. The conclusions of three independent investigators were compared for inter-observer variability assessment. No significant differences were found among the participating centres for electromechanical systole, left ventricular ejection time and pre-ejection time dependence on heart rate and for ratio pre-ejection time-left ventricular ejection time. The regression equations obtained for these indicators were suggested as a national standard for normal subjects.