A method of continuous recording of the disappearance of radioactive materials from the circulating blood is described. By cannulation of the right femoral artery of dogs and leading the tubing past a scintillation counter into the right femoral vein a complete circuit is established in which the concentration of radioactivity may be continuously monitored. Curves showing concentration of the radioactive material versus time have been obtained following injection of iodinated (I131) plasma, Na24, I131, P32, K42, and colloidal Au198. No errors are introduced by sample withdrawal, timing of samples, or geometric variations.