Key points Calcium concentration is kept at extremely low levels inside brain cells; each episode of calcium entry is cleared within seconds. Changes in calcium entry are mediated by spontaneous activity in embryonic mouse brainstem from embryonic day 11.5 to 13.5. Transiently, at embryonic day 12.5, spontaneous events occur frequently such that calcium concentration stays above baseline levels for minutes. This unusual phenomenon, which we termed ‘bash bursts’, is caused by an event that propagates by looping along a defined path; the path gets modified a day later, ending it. The results help us to understand how prolonged increases in calcium concentration can occur in development and how the increases may influence the development of serotonin and dopamine circuits that are related to neurological diseases later in life, such as depression and Parkinson's disease. Abstract Most cells maintain [Ca2+]i at extremely low levels; calcium entry usually occurs briefly, and within seconds it is cleared. However, at embryonic day 12.5 in the mouse brainstem, trains of spontaneous events occur with [Ca2+]i staying close to peak value, well above baseline, for minutes; we termed this ‘bash bursts’. Here, we investigate the mechanism of this unusual activity using calcium imaging and electrophysiology. Bash bursts are triggered by an event originating at the mid‐line of the rostral hindbrain and are usually the result of that event propagating repeatedly along a defined circular path. The looping circuit can either encompass both the midbrain and hindbrain or remain in the hindbrain only, and the type of loop determines the duration of a single lap time, 5 or 3 s, respectively. Bash bursts are supported by high membrane excitability of mid‐line cells and are regulated by persistent inward ‘window current’ at rest, contributing to spontaneous activity. This looping circuit is an effective means for increasing [Ca2+]i at brief, regular intervals. Bash bursts disappear by embryonic day 13.5 via alteration of the looping circuit, curtailing the short epoch of bash bursts. The resulting sustained [Ca2+]i may influence development of raphe serotonergic and ventral tegmental dopaminergic neurons by modulating gene expression.