1. A simple, reliable, and useful indirect method of measuring bone blood flow in animals is described, based on the clearance of a circulating bone-seeking radioisotope. This method is based on an adaptation of the Fick principle. It was originally proposed by Frederickson, Honour, and Copp in 1955. The present authors have amplified the method by developing the standardized techniques, by testing the validity, by studying the possible sources of errors, the advantages and limitations, and by applying the methood to a number of quantitative studies.

2. Bone clearance of the isotope is defined as the volume of blood cleared of the isotope by bone and is obtained by dividing the bone uptake by the average isotope concentration in one milliliter of arterial blood integrated over the first five minutes after injection. The rate of blood flow is expressed as milliliters of blood per minute per 100 grams wet bone (including marrow).

3. The method gives a useful measure of, at best, about 75 per cent of the actual blood flow of bone, and can be applied to a wide variety of studies.

4. A few examples are given. The rates of blood flow of various bones in the rabbit and dog were remarkably similar with an average of about ten milliliters per minute per 100 grams of wet bone including the marrow. The rates of the entire skeletal blood flow in the two species were estimated as about 5 to 10 per cent of the resting cardiac output.

5. One of the major limitations of the method at present is that it is not applicable to man without modification. However, it should be possible to modify the technique so that it could be applied in man at the time of bone biopsy, amputation, or other surgery on bone.