The femoral circulation of living dogs was studied with plastic carbonized microspheres 50 and 200 micra in diameter, labeled with Yterbium169. The femoral blood flow is distributed in the following way: 35% of the particles 50 micra in diameter went to muscle, 45% of the same particles went to skin and 20% to bone. When the particles were 200 micra in diameter 16% went to muscle, 65% to skin and 19% to bone.