Objective The rat BK channel (KCNMA1) splice variant X1 is exclusively expressed in brain and nervous tissue. The widespread BK K⁺ channel is activated by both depolarization and intracellular Ca²⁺. The beta-subunit b2 induces inactivating of BK channels currents in certain brain areas. We aimed at characterizing the X1 variant. Methods A number of BK channel splice variants (Zero, Strex, Slo₂₇, X1) and b2 (KCNMB2) were cloned and expressed in Xenopus oocytes by injection of T7 in vitro transcribed RNA. BK ion- channel properties were observed with two- electrode voltage clamping. Constructs of splice variants were generated by the USER Fusion method in order to include or exclude inserts in the coding sequence. The b2 was also co- expressed with splice variants and constructs. Results Compared to another splice variant Slo27 found in nervous tissue, the X1 showed faster activation kinetics and lower current expression when stimulated by depolarization (up to +120 mV) at low Ca²⁺ concentrations (oocyte intracellular). The X1 variant differs by containing two inserts of 8 (SRTADSLI) and 4 amino acids (SRKR). Deletion of the 8 aa reverted expression from low to high current and retained fast activation, while deletion of the 4 aa insert had little effect. Removal of both inserts slowed activation. X1 co-expressed with b2 showed no inactivation as seen with Slo₂₇ or Zero. The 4 aa insert seemed responsible for this transition. Conclusion Experiments with constructs revealed that the 8 amino acid insert is responsible for the low current expression of X1. The fast activation of X1 seems to depend on both the 8 and 4 amino acid inserts. The 4 amino acid insert is, at least partly, responsible for the non- inactivating phenotype of X1 when co-expressed with b2. Expression of the X1 variant in neurons may play a role for the shaping of action potentials and firing patterns through its faster activation kinetics and sustained current (non-inactivating response to b2).