Radioactive plastic microspheres 50 and 200 micra in diameter labeled with Ytterbium169 were used to study distribution in the femoral circulation of living dogs under general anaesthesia. Shunting was demonstrated because radioactivity was detected in the lungs after intraarterial injection of 50 micra microspheres. When the size of the microspheres injected was 200 micra no radioactivity was detected over the lungs. This indicates that the arteriovenous anastomoses are smaller than 200 micra, they are probably located in the skin because there was less radioactivity in the skin after intrafemoral injection of the 50 micra microspheres than when the injected microspheres were 200 micra in diameter.