The authors have developed a method of measuring placental blood flow using intravenous injection of Xenon 133, the method being derived from that described by Rekonen. The technique is reliable and without any danger. The measurements were taken in the last 4 weeks of 25 normal pregnancies and 12 where intra-uterine growth was retarded. The placental blood flow is 92.02 +/- 16.34 ml s100 ml per minute in normal pregnancies at the end of which infants with a weight above or equal to the 10th percentile were born (the mean percentile was 46.69 +/- 25.36). The placental blood flow was 52.29 +/- 17.31 ml/100 ml per minute in cases with retarded intra-uterine growth where the infants were lighter than the 10th percentile (mean percentile 5.08 +/- 1.83). The difference between the two groups is highly significant (p less than 0.001).