1. In order to determine mechanisms for increased stroke volume at birth, left ventricular function was investigated in eight fetal lambs during in utero pulmonary ventilation. At surgery fetuses were prepared with a tracheal tube, aortic electromagnetic flow sensor, and carotid, jugular, pericardial and left atrial catheters. 2. After 8.3 +/- 1.5 (mean +/- S.D.) post-surgery days and at 137 +/- 3 days gestation, haemodynamic values were obtained before and during pulmonary ventilation in utero. 3. During ventilation, O2 content increased from 7.3 +/- 2.1 to 14.7 +/- 2.7 ml dl-1 (P less than 0.001), right atrial pressure from 3.0 +/- 1.3 to 4.5 +/- 1.6 mmHg (P less than 0.05), left atrial pressure from 2.5 +/- 1.2 to 10.2 +/- 3.7 mmHg (P less than 0.001), left ventricular stroke volume from 1.1 +/- 0.4 to 1.9 +/- 1.2 ml kg-1 (P less than 0.05), and heart rate from 160 +/- 21 to 183 +/- 11 beats min-1 (P less than 0.05). 4. During O2 ventilation, left ventricular function curves relating stroke volume to left atrial pressure were shifted upward. 5. O2 ventilation produced rapid, reversible increases in left ventricular stroke volume which approximate increases in stroke volume at birth.