1. The pulmonary transfer factor for carbon monoxide was measured by the single-breath method in 21 pregnant women with no previous history of cardiac or respiratory disease. Measurements were made at monthly intervals throughout pregnancy and once post partum. 2. The transfer factor was higher in the first trimester of pregnancy than in the non-pregnant state. There was a fall in the transfer factor during pregnancy until 26 weeks gestation, after which no further decrease was observed. 3. The changes in transfer factor were not explained by alterations in haemoglobin concentration or alveolar volume. 4. Simultaneous serial estimation of plasma 17beta-oestradiol were performed in all the subjects. There was no obvious direct relation between changes in the concentration of this hormone and transfer factor measurements.