The purpose of this study was to elucidate the changes in heart rate (HR), systolic and diastolic time intervals accompanying prolonged cycling exercise. Seven healthy male students (N group) and seven male collegiate long distance runners (LDR group) underwent 60-min bicycle ergometer exercise loaded at 30% and 50% HRmax. Electrocardiogram (ECG) and phonocardiogram (PCG) were recorded throughout the exercise and recovery period, and then left ventricular ejection time (LVET) and left ventricular diastolic time (LVDT) were calculated from tracings of the cardiac cycle. In the N group, HR increased to the target HR level (30% and 50% HRmax) in the initial phase of exercise, but there was a tendency to increase 10-15 b/min in the latter half of the exercise period. The LDR group showed the same trend as in the N group at 50% HRmax level (i.e., 120 b/min) exercise. These increments of the HR were due to the decrease of stroke volume, the elevation of body temperature and changes in the volume of the venous return. In the initial phase of exercise (within five minutes), LVDT decreased markedly resulting in a rapid increase of the HR in both groups. The decrease in LVDT was 250-400 msec (60-70% decrement for resting value) at the 30% HRmax level load and 270-480 msec (73-80% decrement for resting value) at the 50% HRmax level load, and then transient slight increment was recognized. Subsequently, there was a tendency to decrease. The major factor for the increase of the HR was that the LVDT decreased markedly that implied the shortening of the inflow time to the left ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)