Microglia are the resident immune cells in the central nervous system and many of their physiological functions are known to be linked to intracellular calcium (Ca 2+ ) signaling. Here we show that isolated and purified mouse microglia -either freshly or cultured -display spontaneous and transient Ca 2+ elevations lasting for around ten to twenty seconds and occurring at frequencies of around five to ten events per hour and cell. The events were absent after depletion of internal Ca 2+ stores, by phospholipase C (PLC) inhibition or blockade of inositol-1,4,5 trisphosphate receptors (IP 3 Rs), but not by removal of extracellular Ca 2+ , indicating that Ca 2+ is released from endoplasmic reticulum intracellular stores. We furthermore provide evidence that autocrine ATP release and subsequent activation of purinergic P2Y receptors is not the trigger for these events. Spontaneous Ca 2+ transients did also occur after stimulation with Lipopolysaccharide (LPS) and in glioma-associated microglia, but their kinetics differed from control conditions. We hypothesize that spontaneous Ca 2+ transients reflect aspects of cellular homeostasis that are linked to regular and patho-physiological functions of microglia.