This study was undertaken to validate the transpulmonary thermodilution cardiac output measurement (CO(TPTD)) in a controlled newborn animal model under various hemodynamic conditions with special emphasis on low cardiac output. Prospective, experimental, pediatric animal study. Animal laboratory of a university hospital. Twelve lambs. We studied 12 lambs under various hemodynamic conditions. Cardiac output was measured using the transpulmonary thermodilution technique with central venous injections of ice-cold saline. An ultrasound transit time perivascular flow probe around the main pulmonary artery served as the standard reference measurement (CO(UFP)). During the experiment, animals were resuscitated from hemodynamic shock using fluid boluses. Cardiac output measurements were performed throughout the experiment. The correlation coefficient between CO(TPTD) and CO(UFP)was .97 (95% confidence interval .94-.98, p < .0001). Bland-Altman analysis showed a mean bias of 0.19 L/min with limits of agreement of -0.04 and 0.43 L/min (12.0% and +/-14.7%, respectively). The correlation coefficient between changes in CO(TPTD) and CO(UFP) during volume loading was .95 (95% confidence interval .91-.96, p < .0001). There was a significant correlation between changes in global end-diastolic volume and changes in stroke volume (r = .59) but not between changes in central venous pressure and changes in stroke volume (r = .03). The transpulmonary thermodilution technique is a reliable method of measuring cardiac output in newborn animals. It is also capable of tracking changes in cardiac output.