We investigated the effects on the electrophysiological properties of ventricular muscle fibres from lizards kept at 20 degrees C of mild and severe hyperthyroidism. The hyperthyroidism was induced by a 4-day treatment with either 0.025 or 1.0 microgram triiodothyronine g-1 body weight, documented by increased serum levels of thyroid hormone. Triiodothyronine treatment did not modify the duration of the action potential recorded in vitro at 25 degrees C from ventricular muscles stimulated at 1 Hz. Recordings at higher temperatures were associated with a faster repolarization phase and a decrease of of action potential duration in both euthyroid and hyperthyroid animals. However, in lizards treated with 1.0 microgram triiodothyronine . g-1 body weight, the 90% repolarization recovery times at 30 and 35 degrees C (95.6 +/- 14.9 ms and 53.0 +/- 6.0 ms, respectively), were significantly shorter than normal (177.6 +/- 29.2 and 107.2 +/ 18.1 ms, respectively). Action potential duration was also dependent on stimulation frequency of the preparations. Increased frequency led to significant decrease of the duration of action potentials recorded at 25 degrees C. In euthyroid preparations the reductions in 90% repolarization recovery time, owing to increase in stimulation frequency to 2.5 and 5 Hz, were 19.3 +/- 1.7 and 35.6 +/- 2.0 ms, respectively. In hyperthyroid preparations, the reductions in the 90% recovery time due to stimulus frequency increases varied from 35.4 +/- 1.9 and 58.1 +/- 2.1 ms at low hormone doses to 38.9 +/- 2.0 and 58.2 +/- 2.1 ms at high hormone doses. As a result of these differences, the action potential durations recorded from the two hyperthyroid preparations at high stimulation rates were shorter than from euthyroid preparations. The results obtained suggest that lizard cardiac tissue is responsive to hormone action at low environmental temperature, but the effects of such action become evident when the temperature and heart rate increase.