The effects of altered thyroid states on the heart rate and ventricular electrophysiological properties of the frog were examined. Hypothyroidism was induced by a 10-day treatment with propylthiouracil and produced decreased serum-free and total triiodothyronine levels below detectable concentrations. Hyperthyroidism, elicited by a 5-day treatment with triiodothyronine, was associated with increased serum thyroid hormone levels. The hypothyroid state was associated with a significantly decreased heart rate measured in vivo and an increased duration of the action potential recorded in vitro from ventricular fibers. Hyperthyroidism was associated with an increased heart rate and a decreased ventricular action potential duration (APD). The dependence of APD on temperature was affected by thyroid status. An increase from 25 to 30 degrees barely shortened the repolarization phase in hyperthyroids, minimally (13.3%) shortened that in euthyroids, and greatly (43.7%) shortened that in hypothyroids; the APD was similar in euthyroid and hypothyroid frogs. The shortening of the repolarization phase, by increased stimulation frequency, was also greater for hypothyroid frogs. In this case, however, the differences in APD among groups remained significant at all the frequencies tested.