We have studied the effects of in vivo alterations of thyroid state on the electrophysiological properties, measured in vitro, of rat papillary muscle fibres. The duration of the action potential, recorded from single cells of papillary muscle obtained from 60 day old rats, thyroidectomized at weaning, was larger than in preparations of euthyroid animals of same age. The treatment of thyroidectomized rats with T3 physiological doses restored the values of action potential duration present in euthyroid animals of same age. The data dealing with the time course of these variations indicate a late effect of thyroidectomy or T3 treatment. The hyperthyroid state, obtained by treatment of thyroidectomized rats with larger doses of T3, was associated with a significant reduction in the action potential. Action potential duration is frequency dependent. As the stimulation rate was increased from 1 to 5 Hz, this duration increased in all groups. However the difference found among the rat groups remained significant. Propranolol had virtually no effect on action potentials from fibres of euthyroid animals and those in altered thyroid state.