Quiescent rabbit left atria were exposed to ouabain for 200 min at various temperatures ranging from 27 degrees to 35 degrees C. Effects produced by the drug were noted when stimulation of the atria was resumed. At or below 31 degrees C, little effect on stimulation was seen and approximately 80 min were required to reach maximum effect of the drug. At and above 32 degrees C, effects could be seen immediately upon stimulation and only 45 min were required to reach maximum effect. In the range from 31 degrees to 32 degrees C, an increment of as little as 0.25 degrees altered both the time to reach maximum effect and the maximum force obtained. Experiments with tritiated ouabain showed no difference in uptake of the drug by atria, whether beating or quiescent, at 30 degrees C or 35 degrees C. Possible mechanisms are discussed.