A method for the continuous recording of uteroplacental blood volume has been described. A radioactive isotope (γ emitter) which is retained in the blood stream is injected into a cubital vein and the organ is surveyed externally using a γ detector. The activity measured is directly proportional to the blood volume surveyed by the detector. The method is satisfactory for organs with a large blood volume compared to that of the tissues intervening between organ and detector. Therefore uteroplacental blood volume changes can be compared with those of other organs. Applications of the method in diagnosis and therapeutics have been discussed.