1. Effects of taurine on the spontaneous action potentials in right atrial muscles of rats were examined using a conventional microelectrode method at 36 degrees C, and at different extracellular Ca2+ concentrations ([Ca]o). 2. When [Ca]o was 1.8 mM, taurine (10 and 20 mM) increased the sinus rate in a concentration-dependent manner. On the other hand, in 5.4 mM [Ca]o solution, 10 mM taurine also increased the sinus rate, but at 20 mM decreased it. 3. Taurine (20 mM) at 1.8 mM [Ca]o decreased the action potential duration, but at 5.4 mM [Ca]o increased it. Other action potential parameters were not altered to a significant extent. 4. In addition, at 5.4 mM [Ca]o, taurine at 10 mM elicited dysrhythmias such as bigeminy, and at 20 mM depressed the spontaneous action potentials and prolonged cycle length. 5. These results indicate that taurine caused a positive chronotropic effect at both [Ca]o levels; however, at 5.4 mM [Ca]o, taurine (20 mM) reversed the positive to a negative chronotropic effect, presumably resulting in the development of cellular calcium overload.