Newborn rabbits were raised in either hypoxic or normoxic environments for 3 months. Data from electrophysiological studies carried out on isolated sinoatrial and ventricular preparations from the two groups show that the hypoxic group had changes in cellular transmembrane electrical activity that could be the basis for disorders of rate and conduction in vivo. This model will be useful to study the etiology and therapy of some arrhythmias seen in cyanotic congenital hypoxemic heart disease. (Pediatr Res 19: 64–66,1985)