Atrial fibrillation (AF) is the most common cardiac arrhythmiaaccounting for significant morbidity and mortality. Cardiac alternans,defined at the cellular level as cyclic beat-to-beat alternationsin cell contraction, action potential morphology, and intracellularCa(2+) concentration, have been associatedwith sustained AF. Although alternans of action potential and intracellularCa(2+) transients in atrial cardiomyocytesin AF have been elucidated, little is known about the mechanicalalternans, cyclic alternations in contraction force, and cardiomyocyte shortening.Moreover, the susceptibility of different regions of atria to alternansformation is unclear. The aim of this study is to analyze and comparethe parameters of alternans in sarcomere shortening-relengtheningin single cardiomyocytes isolated from the left and right atria(LA, RA) of rats with acetylcholine-CaCl2 inducedAF. We found that autonomic AF provoked mechanical alternans insingle LA and RA cardiomyocytes. We also showed that the degreeand magnitude of mechanical alternans were greater in LA cardiomyocytesthan in RA cardiomyocytes. Our results propose that LA cardiomyocyteshave a greater susceptibility to AF-induced beat-to-beat alternans insarcomere shortening amplitudes than RA myocytes that may causeLA mechanical dysfunction.