Synchronized atrial contraction may be much more important in the newborn, who has a faster heart rate and a less compliant ventricle, than in the adult. We therefore investigated the extent by which synchronized atrioventricular contraction contributes to cardiac output and cardiac work in a neonatal circulation, and whether this effect can be fully explained by the Starling mechanism. In neonatal piglets, left ventricular pressure and volume(conductance catheter) were measured during atrial and ventricular pacing. By manipulating preload during atrial pacing, endsystolic pressure and volume, stroke work, and dP/dtmax were compared at the same end-diastolic volume as indices of contractility. Finally, end-diastolic pressure-volume relationships were assessed to investigate the validity of using end-diastolic pressure as an indicator of preload. We found a significant contribution of synchronized atrial contraction; cardiac output increased 27% when pacing mode was switched form ventricular to atrial. The mechanism by which this was achieved is entirely the enhancement of ventricular filling and thus the Starling effect; contractility was unaffected by pacing mode. This large and important effect can be explained by slowed relaxation (compared with the adult ventricle), which impairs passive filling during the ventricular relaxation phase, and makes active filling during atrial contraction more important. In addition, we found that the use of end-diastolic pressure as an indicator of preload, instead of end-diastolic volume, leads to serious misinterpretations, due to not only the nonlinearity of this relationship, but also the possible shifts in this relationship with certain interventions.