In beta cells from Langerhans islets, cell stimulation is coupled to insulin secretion. Upon stimulation with glucose, the concentration of intracellular calcium ions ([Ca2+]i) oscillates in phase with accompanying oscillatory changes in membrane potential and insulin secretion. The calcium oscillations in different cells within a single islet of Langerhans are well synchronized, with phase differences between oscillations in individual cells of up to a few seconds (1, 2). Furthermore, some studies have reported the presence of calcium waves in islets of Langerhans, with a speed of 20-200 μm/s (3). However, many different types of calcium oscillations have been reported in isolated islets which are the most widely used experimental model. It has been suggested that the duration and conditions of islet culture influence the types of responses and that calcium waves occur only in cultured islets (4-6). Additionally, due to limited diffusion of fluorescent calcium indicators, calcium imaging experiments performed on isolated islets enable insight only into the most peripheral parts of islets, where beta cells are least abundant in mice and where other types of cells are present (7). To enable access to all cellular layers of an islet and clarify the existing controversy regarding types of oscillations present in islets and the existence of calcium waves, we combined the recently introduced acute pancreas tissue slice method (8) with confocal laser scanning calcium imaging, using the Oregon green® 488 BAPTA-1 fluorescent calcium indicator. The study was conducted in accordance with all national and European recommendations pertaining to work with isolated tissue and our protocol was approved by the Veterinary Administration of the Republic of Slovenia. The tissue slices were prepared from agarose-injected pancreata of 10-20 week old NMRI mice of either sex, upon sacrifice by cervical dislocation as described in detail previously (8). Taking advantage of previous reports that islet cell types can be identified by their characteristic responses to glucose (7), this enabled simultaneous recording of [Ca2+]i in a large number of cells from all layers of an islet. We showed that fast [Ca2+]i oscillations (4-6 /min) superimposed on a sustained increase in [Ca2+]i are the predominant type of response to 12 mM glucose in beta cells in mice. Additionally, we were able to detect other types of responses, characteristic of alpha and delta cells, predominantly at the periphery of islets. The changes in [Ca2+]i were well synchronized exclusively between coupled beta cells (Figure 1). Finally, employing high speed calcium imaging (20 frames/s) we detected calcium waves spreading in an orderly manner across the beta cell syncitium at a velocity of 80-90 μm/s as the mechanistic substrate of synchronicity between beta cells in uncultured acute pancreas tissue slices (Figure 2). A detailed quantification of activation and deactivation phases before and after the sustained plateau with superimposed oscillations revealed that large amplitude changes in [Ca2+]i are insufficient to ensure intercellular synchronization during stimulation with glucose and that metabolic activation is necessary to align the activity of a heterogeneous population of beta cells.