1The cytoplasmic Ca2+concentration ([Ca2+]i) was measured in single cells and cell clusters of different sizes prepared from mouse pancreatic islets.2During stimulation with 15 mM glucose, 20 % of isolated cells were inert, whereas 80 % showed [Ca2+]ioscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min−1and a less prominent one of 0.27 min−1.3In contrast, practically all clusters (2–50 cells) responded to glucose, and no inert cells were identified within the clusters. As compared to single cells, mean [Ca2+]iwas more elevated, [Ca2+]ioscillations were more regular and their major frequency was slightly higher (but reached a plateau at ≈0.25 min−1). In some cells and clusters, faster oscillations occurred on top of the slow ones, between them or randomly.4Image analysis revealed that the regular [Ca2+]ioscillations were well synchronized between all cells of the clusters. Even when the Ca2+response was irregular, slow and fast [Ca2+]ioscillations induced by glucose were also synchronous in all cells.5In contrast, [Ca2+]ioscillations resulting from mobilization of intracellular Ca2+by acetylcholine were restricted to certain cells only and were not synchronized.6Heptanol and 18α‐glycyrrhetinic acid, two agents widely used to block gap junctions, altered glucose‐induced Ca2+oscillations, but control experiments showed that they also exerted effects other than a selective uncoupling of the cells.7The results support theoretical models predicting an increased regularity of glucose‐dependent oscillatory events in clusters as compared to isolated islet cells, but contradict the proposal that the frequency of the oscillations increases with the number of coupled cells. Islet cell clusters function better as electrical than biochemical syncytia. This may explain the co‐ordination of [Ca2+]ioscillations driven by depolarization‐dependent Ca2+influx during glucose stimulation.