We used intracellular microelectrodes to study the effect of changes in extracellular calcium ion concentration [Ca2+]oon the transmembrane potentials of canine cardiac Purkinje fibers in control Tyrode's solution and in the presence of agents thought to modify membrane permeability to potassium. In Tyrode's solution, decreasing [Ca2+]ofrom 2.7 to 0.9 mM increased action potential duration measured at 60 mv (APD «,) and at full repolarization (APDioo) but did not significantly modify the normal linear relationship between cycle length and APD between cycle lengths of 500 to 4000 msec. We used 9-aminoacridine (9-AA) to decrease potassium permeability. At concentrations between 0.01 and 1.5 x 10"5M, 9-AA caused a concentration-dependent increase in APD-Mand APD1Oo and a significant increase in the slope of the line relating APD to cycle length. The usual effects of changes in [Caz+]oon APD were potentiated by 9-AA. We then used lidocaine (L) to increase potassium permeability. At concentrations between 0.75 and 1.5 x 10~5M, L significantly increased the slope of the line relating APD and cycle length. In the presence of L an increase in [Ca2+]oincreased both APD_eo and APD,oo, and a decrease in [Ca2+]odecreased both parameters. 9-AA thus potentiated and L reversed the effects of changing [Ca2+]oon APD. The results suggest that changes in [Ca2+]omodify repolarization by modifying both slow inward current and potassium permeability and that the extent to which the latter changes can determine the effect of [Ca2+]oon APD. Ore Res 46: 227-236, 1980