1. Hitherto, no study has been reported on the relative quantitative contributions of blood supply by the different arterial systems of long bone. This paper is a report on such a study in the young adult rabbit.

2. The rates and regional distributions of the blood supply of the nutrient as well as other arteries of the femur were studied after ligation of the nutrient artery. The average rates of reduction in blood flow per minute for the first five minutes through the entire femur as well as the shaft, and the epiphysis and metaphysis on each end, were measured and analysed. The bone blood flow was measured by the method of bone clearance of blood strontium 85.

3. The normal average rate of blood flow through the femurs of average weight of 9·38 grammes was 0·90±0·05 millilitres per minute, or 9·60±0·47 millilitres per minute per 100 grammes of bone.

4. The nutrient artery contributed at least 46 per cent of the normal total blood supply of the entire femur and at least 71 per cent of the normal total blood flow of the shaft including its marrow, and 37 per cent and 33 per cent of the normal total blood flow of the upper and the lower epiphysial and metaphysial areas respectively.

5. About 63 per cent, 30 per cent and 67 per cent of the total normal blood flow through the upper epiphysis and metaphysis, the shaft and the lower epiphysis and metaphysis respectively are still intact in the first five minutes after ligation of the nutrient artery, which represent the approximate proportions of the blood supply by the other regional arteries.

6. These quantitative data obtained in this study offer good support to the qualitative observations made by many previous workers.