Background: Action potential (AP) contours vary considerably between normal adult and aged right atrial fibers. The ionic bases for these differences remain unknown. Therefore we studied Ca 2 + and K + currents in cells from adult and aged canine right atria (RA). Methods and Results: We used whole cell patch clamp recording techniques to measure L-type Ca 2 + currents (I C a L ) with either Ca 2 + or Ba 2 + (3 mM) as the charge carrier, and both the transient outward (I t o ) and sustained potassium currents (I s u s ) in cells dispersed from normal adult (Adult, 2-5 years) and older dogs (Aged, >8 years). There is a significant reduction in peak I C a L (47%) and I B a L (43%) in Aged cells, yet differences in I B a L disappear with maximal β adrenergic stimulation (isoproterenol, 1 μM). Composite I t o and I s u s densities were significantly increased in the Aged versus Adult cell group (by 31 and 27% at +50 mV, respectively). I t o decay during a maintained depolarization was slowed in Aged cells. Furthermore, I t o steady-state inactivation curve was shifted positively in Aged cells. Finally, composite I t o and I s u s currents of Aged cells were more sensitive to tetraethylammonium chloride (TEA), a specific inhibitor of some types of K + currents. In the presence of TEA (5 mM), I t o in Aged cells was significantly greater than that in Adult cells. Conclusions: Ionic currents differ in Aged versus Adult right atrial cells, such that a reduced Ca 2 + current and augmented outward currents could contribute significantly to the altered AP contour of the Aged RA cell. Adrenergic stimulation appears to restore Ba 2 + currents in Aged cells. Finally, an augmented TEA sensitive current plays a role in changes of I s u s in Aged right atrial cells.