Introduction: Tachycardia-induced cardiomyopathy (TCM) is a reversible form of heart failure. However, the effects of persistent tachycardia leading to TCM in the human are less understood limiting appropriate management of patients with this underrecognized condition. This study investigated the electrophysiological mechanisms of TCM. Methods and Results: Tachycardic in vitro electrical stimulation (120 bpm versus 60 bpm control) of human specimen was performed. In human twitching ventricular trabeculae from patients with preexistent heart failure, tachycardia for 8h compromised systolic force, while diastolic tension and relaxation time were increased (n=6/8 patients each). Human iPSC-cardiomyocytes (iPSC-CM) of healthy donors were utilized as human model suitable for extended electrical pacing. After 24h of tachycardia, systolic Ca 2+ -transient amplitude of iPSC-CM was reduced, while diastolic Ca 2+ levels and cytosolic Ca 2+ elimination were not changed (Fura-2, n tachycardia/control=49/44 cells). 7d of tachycardia resulted in a progressive decline of Ca 2+ -transient amplitude together with an impaired Ca 2+ elimination (n=73/66) thereby demonstrating the progression of TCM. Sarcoplasmic reticulum (SR) Ca 2+ load (caffeine application) and SERCA activity (K sys -K caff ) were reduced after 7d of tachycardia (n=13/13). Besides an impaired SERCA activity, we could identify an increased diastolic SR Ca 2+ spark frequency after 7d of tachycardia (n=76/79) as another mechanism of reduced SR Ca 2+ load. In voltage clamp studies, I CaL density was reduced after 7d of tachycardia (n=11/12), which may further contribute to impaired systolic Ca 2+ -handling. Conversely, action potential duration was prolonged after 7d of tachycardia compared to control (n=19/21). As a potential explanation for prolonged action potentials however, late sodium current (I NaL ) was enhanced after 7d of tachycardia (n=26/19). Conclusions: This study demonstrates for the first time in human myocardium that persistent tachycardia induces early adverse electrophysiological remodeling that may underlie the progression of TCM. These findings may help to understand the pathophysiology of an underrated but prevalent disease