In contrast to pancreatic islets, isolated -cells stimulated by glucose display irregular and asynchronous increases in cytoplasmic Ca²⁺ ([Ca²⁺]_i). Here, clusters of 5-30 cells were prepared from one single mouse islet or from pools of islets, loaded with fura-2 and studied with a camera-based system. [Ca²⁺]_i oscillations were compared in pairs of clusters by computing the difference in period and a synchronization index . During perifusion with 12mM glucose, the clusters exhibited regular [Ca²⁺] oscillations that were quasi perfectly synchronized ( period of 1.4 % and index close to 1.0) between cells of each cluster. In contrast, separate clusters were not synchronized even when prepared from one single islet. Pairs of clusters neighbouring on the same coverslip were not better synchronized than pairs of clusters examined separately (distinct coverslips). We next attempted to synchronize clusters perifused with 12mM glucose by applying external signals. A single pulse of 20mM glucose, 10mM amino acids or 10µM tolbutamide transiently altered [Ca²⁺]_i oscillations but did not reset the clusters to oscillate synchronously. On a background of 12mM glucose, repetitive applications (1min/5min) of 10µM tolbutamide but not of 20mM glucose synchronized separate clusters. Our results identify a level of -cell heterogeneity intermediate between single -cells and the whole islet. They do not support the idea that substances released by islet cells serve as paracrine synchronizers. However, synchronization can be achieved by an external signal, if this signal has a sufficient strength to overwhelm the intrinsic rhythm of glucose-induced oscillations and is repetitively applied.