To evaluate timolol's ability to antagonize the "slow response" in cardiac tissues, isolated canine cardiac Purkinje fibers were exposed to 0.25 mM barium chloride to induce membrane depolarization, spontaneous automaticity, and slow response action potentials. The effects of timolol on action potential parameters and the cycle length of spontaneous automaticity were tested at concentrations of 10, 100, and 500 micrograms/ml. Timolol significantly depressed the action potential and slowed automaticity only at 100 and 500 micrograms/ml but not at 10 micrograms/ml. Since verapamil and nifedipine have been reported to completely suppress activity in this model at concentrations of 0.2-2.0 micrograms/ml, we conclude that timolol has little direct slow response blocking potency.