The effects of cycle length (CL) alteration on action potentials of isolated trabeculae of the common right atrial working myocardium (M) and isolated preparation of the crista terminalis (CT) of the rabbit heart were studied. Prolonging the CL increased the steady state duration of action potentials in M but decreased it in CT fibres. The repolarization of the post-pause action potentials of the M preparations was characterized by (i) a rapid initial repolarization followed by (ii) a plateau phase and (iii) a retarded terminal repolarization. In CT fibres the retarded repolarization was not observed and the duration of the post-pause action potentials at 90% repolarization is decreased. Premature action potentials of the M preparations were shortened at 90%, but were prolonged at 25% repolarization. Premature action potentials of the CT fibres were prolonged. The post-drive resting transmembrane potential of the M preparations moved to more negative values depending on the CL stimulation. The shift of the post-drive transmembrane resting potential to negative values in CT preparations was preceded by an initial depolarization. The initial depolarization was absent in M preparations. Myofibril-poor pale cells were found consistently in CT, but never in M preparations.