In the present work, we found that the delayed rectifying outward potassium current (I(K)) in adult and neonatal cat ventricular myocytes consists of both rapid and slow components, I(Kr) and I(Ks), respectively, which can be isolated pharmacologically. Thus after complete blockade of I(Kr) with dofetilide, the remaining I(Ks) current is homogeneous, as shown by an envelope of tails test. I(Kr) maximum tail current density, measured at -40 mV, was similar in adult and neonatal myocytes. I(Ks) maximum tail current density in neonatal myocytes, measured at -40 mV, was significantly smaller than in adult myocytes. Activation kinetics of I(Kr) and I(Ks) was similar in both age groups. However, the I(Kr) deactivation time course was significantly faster in neonatal than in adult myocytes. Developmental differences in the subunit composition of I(Kr) that display distinctly different deactivation kinetics are suggested.