1. Voltage-clamp studies on atrial myocytes isolated from adult and postnatal day 15 (P15) C57BL6 mice demonstrate the presence of three kinetically distinct Ca2+-independent, depolarization-activated outward K+ currents: a fast, transient outward current (Ito,f), a rapidly activating, slowly inactivating current (IK,s) and a non-inactivating, steady-state current (Iss). The time- and voltage-dependent properties of Ito,f, IK,s and Iss in adult and P15 atrial cells are indistinguishable. 2 Pharmacological experiments reveal the presence of two components of IK,s: one that is blocked selectively by 50 μM 4-aminopyridine (4- AP), and a 4-AP-insentive component that is blocked by 25 mM TEA; Iss is also partially attenuated by 25 mM TEA. There are also two components of IK,s recovery from steady-state inactivation. 3 To explore the molecular correlates of mouse atrial IK,s and Iss, whole-cell voltage-clamp recordings were obtained from P15 and adult atrial cells isolated from transgenic mice expressing a mutant Kv2.1 α subunit (Kv2.1N216Flag) that functions as a dominant negative, and from P15 atrial myocytes exposed to (1 μM) antisense oligodeoxynucleotides (AsODNs) targeted against Kv1.5 or Kv2.1. 4 Peak outward K+ current densities are attenuated significantly in atrial myocytes isolated from P15 and adult Kv2.1N216Flag-expressing animals and in P15 cells exposed to AsODNs targeted against either Kv1.5 or Kv2.1. 5 Analysis of the decay phases of the outward currents evoked during long (5 s) depolarizing voltage steps revealed that IK,s is selectively attenuated in cells exposed to the Kv1.5 AsODN, whereas both IK,s and Iss are attenuated in the presence of the Kv2.1 AsODN. 6 In P15 and adult Kv2.1N216Flag-expressing atrial cells, mean ± S.E.M. IK,s and Iss densities are also significantly lower than in non-transgenic atrial cells. In addition, pharmacological experiments reveal that the TEA-sensitive component IK,s is selectively eliminated in P15 and adult Kv2.1N216Flag-expressing atrial cells. 7 Taken together, the results presented here reveal that both Kv1.5 and Kv2.1 contribute to mouse atrial IK,s, consistent with the presence of two molecularly distinct components of IK,s. In addition, Kv2.1 contributes to mouse atrial Iss.