Intervillous blood flow (IVBF) was measured intravenously with a new quantitative 133Xe method in 50 normal and 74 complicated late pregnancies between the 35th and 42nd weeks. The distribution of individual flow rates seemed to be fairly wide in both the normal and the pathological groups. The mean rate of IVBF in normal pregnancies was 140 ml/100 ml of intervillous space/min. The lowest mean flow values were observed in pregnancies complicated by diabetes mellitus (class B-E), cholestasis of pregnancy and severe pre-eclampsia, a highly significant difference (p less than 0.001) from the mean IVBF observed in normal pregnancies. The significance of the results in the different groups has been discussed in detail. This method may open up a new diagnostic area in the management of high-risk pregnancies.