The striatum plays an important role in linking cortical activity to basal ganglia output. We conducted the calcium (Ca2+) imaging to investigate the spontaneous activities of the striatum using acute slice preparations. Corticostriatal slices of rat brain were stained with Fura-PE3-AM. Long-lasting spontaneous intracellular Ca2+ ([Ca2+]i) transients, which lasted up to about 250 s, were observed. The amplitudes of the transients were variable even in a single cell. Most cells exhibited irregular frequencies, but some exhibited oscillatory features. These [Ca2+]i transients were not induced by action potentials because they were not inhibited by tetrodotoxin. Antagonists of the ionotropic glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione and D,L-2-amino-5-phosphonovaleric acid, did not block these transients. These results suggested that the action potentials and the excitatory synaptic inputs in these striatal network were not involved in the induction of the [Ca2+]i transients. In contrast, the number of the active cells, which exhibited the [Ca2+]i transients, was greatly reduced by the intracellular Ca2+ store depletor, thapsigargin. Therefore, the intracellular Ca2+ store is likely to contribute to the [Ca2+]i transients.