Taurine (2-aminoethane sulfonic acid) is found in high concentrations in the heart, particularly in Purkinje fibers. We studied the transport of taurine in Purkinje fibers that were excised rapidly from the heart and placed in a vessel containing oxygenated Krebs-Henseleit solution (37 °C). After equilibration, 4.4X10−6M radiolabeled taurine[MC] was added to the bath. A computer compart-mental analysis of the uptake and efflux indicated the presence of two pools for uptake—a pool with a rapid kinetics Ki (ti/2= 0.80 min) and K2(t|/2= 176.30 min). These studies suggest that Purkinje fibers have the capacity to transport taurine rapidly. Michaelis-Menten procedures showed the presence of a high affinity and a low affinity transport process. Guanidinotaurine, at a 10:1 ratio, had no appreciable effect on taurine uptake, but 3-aminopropane phosphonic acid decreased taurine uptake by 42.7%. Ouabain and acetylstrophanthidin (10−5M) inhibited taurine uptake (Ki) by 34% and 73%, respectively. The inhibition of the rapid component of taurine uptake suggests that Ki is an energy-linked process possibly requiring Na+,K+-ATPase. Taurine uptake in a calcium-free medium was decreased by 58%. Verapamil (6 x 10"6M) decreased taurine uptake by 42%. Tetrodotoxin (3.4 x 10"5M) decreased taurine uptake by 51%. The requirement of calcium and sodium for taurine uptake suggests an important relationship between taurine, calcium, and sodium in the function of fibers in the cardiac conducting system.Circ Res 47: 763-769, 1980