Abstract —Inward rectification, an important determinant of cell excitability, can result from channel blockade by intracellular cations, including Ca ²⁺ . However, mostly on the basis of indirect arguments, Ca ²⁺ -mediated rectification of inward rectifier K ⁺ current ( I _K1 ) is claimed to play no role in the mammalian heart. The present study investigates Ca ²⁺ -mediated I _K1 rectification during the mammalian ventricular action potential. Guinea pig ventricular myocytes were patch-clamped in the whole-cell configuration. The action potential waveform was recorded and then applied to reproduce normal excitation under voltage-clamp conditions. Subtraction currents obtained during blockade of K ⁺ currents by either 1 mmol/L Ba ²⁺ ( I _Ba ) or K ⁺ -free solution ( I _0K ) were used to estimate I _K1 . Similar time courses were observed for I _Ba and I _0K ; both currents were strongly reduced during depolarization (inward rectification). Blockade of L-type Ca ²⁺ current by dihydropyridines (DHPs) increased systolic I _Ba and I _0K by 50.7% and 254.5%, respectively. β-Adrenergic stimulation, when tested on I _0K , had an opposite effect; ie, it reduced this current by 66.5%. Ryanodine, an inhibitor of sarcoplasmic Ca ²⁺ release, increased systolic I _Ba by 47.7%, with effects similar to those of DHPs. Intracellular Ca ²⁺ buffering (BAPTA-AM) increased systolic I _Ba by 87.7% and blunted the effect of DHPs. Thus, I _K1 may be significantly reduced by physiological Ca ²⁺ transients determined by both Ca ²⁺ influx and release. Although Ca ²⁺ -induced effects may represent only a small fraction of total I _K1 rectification, they are large enough to affect excitability and repolarization. They may also contribute to facilitation of early afterdepolarizations by conditions increasing Ca ²⁺ influx.