Intracellular Ca2+ augments delayed rectifier K+ current (IK) in cardiac myocytes, which may play a major modulatory role in repolarization of action potentials. We investigated subcellular mechanisms for Ca2+-induced enhancement of IK in large-pipette inside-out membrane patches excised from isolated guinea pig ventricular myocytes. When [Ca2+]i was raised from 10−8 to 10−6 mol/L, the amplitude of IK measured at +80 mV was increased from 12.0±2.2 to 19.5 ± 3.3 pA (P<.01). The enhancement of IK by Ca2+ was dose dependent, with an EC50 of 3.8× 10−8 mol/L. A calmodulin antagonist, W7 (50 μmol/L), calmidazolium (100 μmol/L), or HT-74 (20 μmol/L), added to the intracellular solution abolished enhancement of IK by Ca2+, whereas the inactive form of the W7 analogue, W5, had no effect on IK. In the presence of a protein kinase inhibitor with a relatively high specificity for protein kinase C (H7), for protein kinase A (H8 or peptide-type inhibitor PKI), or for calmodulin kinase II (KN-62) or a nonspecific inhibitor of serine/threonine protein kinases (staurosporine), increases in [Ca2+], still enhanced IK. Ca2+-induced enhancement of IK was also observed when Mg2+ and ATP were omitted from the intracellular solution to delete exogenous phosphate donors and when adenylylimidodiphosphate was added to preclude trapped cytoplasmic substrates. Thus, cardiac IK was enhanced by increases in [Ca 2+]i at a physiological range via a calmodulin-dependent pathway, which did not involve a phosphorylation process.