The mouse brain K+ channel (MBK), previously cloned by others, has been independently cloned and shown to express in Xenopus oocytes. This K+ current (IK) inactivated over a time course of seconds and was sensitive to the K+ channel-blocking reagent tetraethylammonium. When the K+ channel was coexpressed with a cloned mouse brain serotonin receptor (5HT1c) in oocytes, activation of the 5HT1c receptor by a brief application of serotonin resulted in a suppression of the IK amplitude over the next 20 min. IK could also be suppressed by activation of G proteins. Suppression was also caused by intracellular Ca2+ injections and was blocked by intracellular injection of EGTA. Calmodulin antagonists block the IK suppression, but a known protein kinase inhibitor did not block suppression. The 5HT1c suppression was reversible; recovery from suppression was blocked by the protein kinase inhibitor H-7. These data suggest that the IK suppression occurs through a novel mechanism independent of A- or C-type protein kinases; suppression is best explained as being due to the action of a Ca2+/calmodulin-activated phosphatase; recovery from suppression is due to the action of a protein kinase.