We report our findings in a chronic canine study performed to evaluate the effects of thyroid funciton alteration on atrial and ventricular pacing parameters. Each of five dogs with normal, healthy myocardia, received four unipolar Medtronic contempory pacing leads - model 4011 Target Tip and model 4023 Capsure SP ventricular leads, model 6957 atrial screw-in and an additional lead, either model 4523 Capsure SP or model 4511 Target Tip passive fixation lead. Baseline electrical data and blood levelof T3, T4, electrogram amplitude and slew rate in both the atrium and the ventricle for each electrode type studied during hypothyroid vs hyperthyroid states. Minor decreases in ventricular thresholds were identified. Hyperthyroid induction did not chage either sensing or stimulation parameters from the euthyroid vaseline values.