The xenon washout technic has been assessed as a method for measuring placental blood flow in humans. In the first group of 20 patients, the 133Xe injection was made blindly into the placenta; in the second group of 20 patients, a new technic was employed to ensure injection into the vascular (intervillous) space. The results have been calculated as clearance half-times, in seconds. Both groups gave widely differing results. After comparing the clearance half-times for 133Xe with those for 99mTc-albumin, it was concluded that with intervillous 133Xe injections, a true xenon washout effect does not occur. Thus, calculations for blood flow in ml/g tissue/unit time are not considered valid. Further, the method cannot be recommended as a test for the action of drugs on placental blood flow. Firstly, under the influence of spontaneous placental pressure variations, single measurements are of limited value. Secondly, as blood examinations have shown the needle puncture itself may damage fetal vessels, permitting possibly dangerous fetomaternal transfusion.