Transmembrane potentials were recorded from sinoatrial node and muscle cells in atria isolated from euthyroid, hypothyroid and thyrotoxic rabbits. Compared to euthyroid atria, thyrotoxic atria had a higher rate (mean 176 versus 118 beats/min). This was the result of an increased rate of diastolic depolarization (mean 86 versus 55 mV/sec) and a decreased duration of the action potential (mean 72 versus 105 sec) in the sinoatrial node cells. Hypothyroidism has the opposite effect; rate 82 versus 118 beats/ min; diastolic depolarization 35 versus 55 mV/sec; action potential duration 125 versus 105 msec. Changes in atrial rate or in the release of endogenous neurotransmitters could not account for the alterations in transmembrane potentials seen in the various thyroid states. Since both diastolic depolarization as well as repolarization of the action potential in sinoatrial cells are believed to be related to changes in membrane conductance for potassium, thyroid hormone may selectively and directly influence these changes.