Objectivs: BK and SK4 channel knockout (KO) mice were used to examine the role of K_Ca channels in glucose homeostasis, beta-cell function and viability. Methods: Electrophysiological experiments were performed with the patch-clamp technique. [Ca²⁺]_c was determined by fura-2, insulin release by RIA, apoptotic cell death via activated caspase3. Results: Glucose-tolerance tests of both KO-mouse strains revealed opposed results: SK4-KO mice elicited a lower blood glucose concentration compared to WT littermates. In contrast, BK-KO mice were less glucose tolerant than WT mice. The improved glucose tolerance of SK4-KO mice can be explained by changes in the stimulus-secretion coupling (SSC) which can be mimicked in WT cells by the SK4 channel blocker TRAM-34. Glucose-induced action potential (AP) frequency was increased in SK4-KO beta-cells. This increase in electrical activity was mirrored by enhanced [Ca²⁺]_c. Likewise, TRAM-34 was able to increase AP frequency and [Ca²⁺]_c. At a threshold glucose concentration without electrical activity TRAM-34 depolarized V_m and induced APs. Similarly, the glucose concentration curve for electrical activity was left-shifted in SK4-KO beta-cells vs WT cells. In addition, the glucose responsiveness of insulin secretion in SK4-KO islets was shifted to lower glucose concentrations. In BK-KO islets glucose-induced insulin secretion was reduced. However, other parameters of SSC were not different between WT and BK-KO beta-cells. Measuring apoptosis revealed that BK-KO mice had a higher rate of apoptotic islet cells compared to WT littermates. Conclusion: KO or inhibition of the SK4 channel increases the sensitivity of beta-cells to glucose. This makes the SK4 channel an interesting new target for anti-diabetic drugs. The BK channel seems to be only marginally involved in the SSC of beta-cells, at least in mice, but it may play an important role in beta-cell survival.