1. The uptake of an intra-arterial injection of 18F was compared with the uptake of a simultaneous injection of 51Cr-labelled microparticles in rabbit bone. 2. The mean single-passage extraction ratio of 18F was 102% (sem 11%, n = 9) in the femur and 119% (sem 9%, n = 10) in the tibia. 3. A mean of 41% (sem 3%, n = 9) of the afferent blood to the femur and 28% (sem 4%, n = 10) to the tibia was found to flow to the marrow. 4. All the efferent blood from the marrow was cleared of 18F by bone. 5. It is concluded that 18F is suitable as a tracer for the measurement of bone blood flow in man and as a reference tracer for measuring the extraction ratios of other bone-seeking radiopharmaceuticals.