The vascular anatomy of the canine femur was studied with particular attention to the venous efflux systems. A new technique was described which makes possible the collection of venous blood from the canine femur in acute as well as in chronic, unanesthetized preparations. In the latter preparation, two polyethylene catheters were inserted through side branches into segments of the femoral vein that received blood from each end of the femur but were otherwise isolated by snares above and below so that these isolated segments served as reservoirs which were drained by the catheters. The methods described have been used to measure the rate of blood flow through the femur, the differences in concentration of blood consitutents in the arterial blood and in the venous blood draining from the femur, and the net flux of various plasma constituents across the intact canine femur.