We have studied the effects of thyroidectomy and the in vivo administration of different triiodothyronine (T₃) doses in thyroidectomized female rats on electrophysiological properties, measured in vitro, of the anterior and posterior papillary muscle fibers from the right ventricle. In each thyroid state, the action potential duration (APD) measured by stimulating at 1 Hz was shorter for the posterior papillary muscle. APD from both preparations was found significantly lengthened in thyroidectomized animals in comparison to euthyroid controls. APD was shortened owing to treatment of thyroidectomized rats with T₃ doses up to 10 µg/100 g body weight every second day. Treatment with larger doses of T₃ tended to restore the values of APD found for ventricular fibres from both controls and thyroidectomized animals treated with substitutive T₃ doses (5 µg/100 g body weight every second day). As the stimulation rate was increased from 1 to 5 Hz, APD increased in both preparations of all groups. The changes were of different amounts but the APD difference between the rat groups, which were significant at 1 Hz, remained significant at 5 Hz, while the differences between anterior and posterior preparations were cancelled in animals treated with 50 µg of T₃ and reversed in those treated with 100 µg.