The method for blood flow determination involving measurement of relative radioemission from radioactive microspheres retained in various tissues after intravascular injection has here been applied to minute tissue regions. 85Sr-labelled plastic microspheres with a diameter of 15 ± 5 μ were injected in a retrograde manner into the aorta in 10 anesthetized adult rabbits. The tissues included in the present study were the m. quadriceps femoris tendon, the cortical bone and the corresponding bone marrow from the left femur diaphysis. The unknown blood flows through these tissue regions were calculated by relating the measured radioactivity from tissue samples to a known blood flow and a known blood radioactivity in cannulated branches from the left posterior tibial artery. The blood flow values estimated by this technique were (g/minxg tissue): Quadriceps femoris tendon -0.068 ± 0.044, bone marrow of femur diaphysis -0.250 ± 0.146, cortical bone of femur diaphysis -0.010 ± 0.007.