1. In adult rat atrial myocytes, three kinetically distinct Ca2+-independent depolarization-activated outward K+ currents, I(K,fast), I(K,slow) and I(SS), have been separated and characterized. 2. To test directly the hypothesis that different voltage-dependent K+ channel (Kv channel) α subunits underlie rat atrial I(K,fast), I(K,slow) and I(SS), the effects of antisense oligodeoxynucleotides (AsODNs) targeted against the translation start sites of the Kv α subunits Kv 1.2, Kv 1.5, Kv 4.2, Kv 4.3, Kv 2.1 and KvLQT1 were examined. 3. Control experiments on heterologously expressed Kv α subunits revealed that each AsODN is selective for the subunit against which it was targeted. 3. Peak outward K+ currents were attenuated significantly in rat atrial myocytes exposed to AsODNs targeted against Kv 4.2, Kv 1.2 and Kv 1.5, whereas AsODNs targeted against Kv 2.1, Kv 4.3 and KvLQT1 were without effects. 5. No measurable effects on inwardly rectifying K+ currents (I(K1) were observed in atrial cells exposed to any of the Kv α subunit AsODNs. 6. Kinetic analysis of the currents evoked during long (10 s) depolarizing voltage steps revealed that AsODNs targeted against Kv 4.2, Kv 1.2 and Kv 1.5 selectively attenuate rat atrial I(K,fast), I(K,slow) and I(SS), respectively, thus demonstrating that the molecular correlates of rat atrial I(K,fast), I(K,slow) and I(SS) are distinct. 7. The lack of effect of the Kv 4.3 AsODNs on peak outward KC currents reveals that Kv 4.2 and Kv 4.3 do not heteromultimerize in rat atria in vivo. In addition, the finding that Kv 1.2 and Kv 1.5 contribute to distinct K+ currents in rat atrial myocytes demonstrates that Kv 1.2 and Kv 1.5 also do not associate in rat atria in vivo.