The roles of calmodulin (CaM)-dependent protein kinase II (CaMKII) in the maintenance of basal activity and the reversion of run-down of L-type Ca²⁺ channels were studied in guinea-pig ventricular myocytes by the patch-clamp technique. In the cell-attached configuration, the Ca²⁺-channel activity was inhibited to 82% – 26% by 1 – 10 μM KN-93 and to 92% – 66% by 0.1 – 1 μM autocamtide-2–related inhibitory peptide (AIP) myristoylated. In the inside-out configuration, the bovine cardiac cytoplasm recovered Ca²⁺-channel activity to 87% of that recorded in the cell-attached configuration, while the CaMKII inhibitor 281-301 at 10 μM reduced the recovery effect to 19%. CaM + ATP recovered the channel activity to 93% and 28% of that recorded in the cell-attached configuration when applied at 1 and 5 min after run-down, respectively, showing a time-dependent attenuation. However, in the presence of 0.33 μM CaMKII, this attenuation was abolished, showing 85% and 75% recovery when applied at 1 and 5 min after run-down, respectively. This recovery effect was suppressed by 10 μM AIP, applied at 5 min, but not at 1 min after run-down. We concluded that CaMKII activation is required in the maintenance of basal activity of L-type Ca²⁺ channels.