This study was undertaken to determine (1) the role of the lymphatic system in the mobilization and transport of intramusc. injected radioactive Na; (2) the accuracy of the external measurement of radioactive Na removal as compared to direct detns. made on blood draining from the injected muscle. Thoracic duct fistulas were made in 6 dogs and the duct cannulated in 1, and chyle was collected for periods up to 3 hrs. after injn. of radioactive Na into one of the gastrocnemius muscles. By use of special electromechanical apparatus and counting technics the percentage of injected radioactive Na present in the collected chyle was determined. This value was found to be 1.1% or less in all of the animals studied. It was concluded that the lymphatic system of the dog''s hind extremity does not play a significant role in the mobilization of intramusc. injected Na; nor does the radioactive material diffuse along intramusc. planes beyond the range of the Geiger-Muller counter. The radioactive Na content of the blood draining from the gastrocnemius muscle was detd. in 11 instances as follows: after first occluding all vessels of the extremity, except the femoral artery and vein, with a malleable wire tourniquet, the femoral vein was sectioned, and all of the blood moving cephalad via the distal half was collected. Blood from healthy mongrel dogs was quantitatively replaced through the proximal end of the femoral vein during the phlebotomy. The amt. of isotope known to have been mobilized from the gastrocnemius muscle (as measured by a Geiger-Muller counter beneath this muscle) was quantitatively recovered in the blood collected from the femoral vein. The rate of Na removal measured external to the site of injn. was the same as that detd. from the blood actually draining from the muscle.