To determine the effect of respiration on systolic and diastolic time intervals, simultaneous phonocardiograms, carotid pulse tracings, M mode echocardiograms, and respiratory curve tracings were measured in 25 healthy subjects. The positioning of each cardiac cycle in relation to the phase of respiration was assessed and the dependency of heart rate and cardiac time intervals on respiration was examined. Heart rate clearly varied over the respiratory cycle. Where necessary the time intervals were corrected for heart rate or RR interval. The systolic time intervals showed a stronger dependency on respiratory group than the diastolic time intervals. The decrease in left ventricular ejection time and increase in pre-ejection period and isovolumic contraction time during inspiration support the idea that a relative increase in afterload in inspiration determines left ventricular systolic function. Isovolumic relaxation time also showed cyclic behaviour whereas the left ventricular filling time was affected by inspiration only. Filling time increased significantly when there was a transition from expiration to inspiration during left ventricular ejection. It seems that when isovolumic contraction takes place in expiration the diastolic intervals of this cycle take on an expiratory character. The increase in filling can be viewed as a compensatory effect that partly offsets the loss of stroke volume during inspiration.