Seventy-one pregnant rats of a Sprague-Dawley strain were exposed for a single 24-hour period to a gas mixture of 6 per cent carbon dioxide, 20 per cent oxygen and 74 per cent nitrogen on different days during gestation. There were 530 offspring and these were compared with 159 newborn rats from 21 control dams. The incidence of cardiac malformations was 23.4 per cent in the test group and 6.8 per cent in the control group with the highest incidence in the test group occurring when the exposure was on the tenth day of gestation. Isolated ventricular septal defects, situated in the lower and posterior part of the muscle septum, were found in 8 cases, ventricular septal defects with overriding aorta in 24 cases and ventricular septal defects with partial transposition in 20 cases. Forty-seven of the abnormal hearts had a myocardial thickening accompanied by narrowing of the pulmonic or aortic outflow channel with or without other abnormalities, such as atresia of the mitral or tricuspid valves and hypoplasia of the aorta or pulmonary artery. There was a slight increase in perinatal mortality in the test group, and a lower frequency of male offspring. The average body weight was 18.9 per cent higher in the test group.