It has been reported that frequency of action potential firing of striatal cholinergic interneurons increases with postnatal development. On the other hand, changes in spontaneous firing frequency and firing properties during aging have not been investigated. In this study, cell-attached and whole-cell patch-clamp studies were carried out to investigate changes in firing properties of striatal cholinergic interneurons during aging. Brain slices were prepared from 2–3-month-old, 11–12-month-old and 24-month-old mice of either sex. Frequencies of spontaneous firing at 2–3-month-old, 11–12-month-old and 24-month-old were 4.55 ± 1.01 Hz (n = 18), 8.73 ± 2.28 Hz (n = 8) and 2.88 ± 0.99 Hz (n = 11), respectively. Firing frequency at 14 month of age was significantly decreased than that of 11–12-month-old (p = 0.028). Since spontaneous firing appeared to be irregular in 24-month-old mice, the coefficient of variation (CV) of the inter-event interval of spontaneous firing was analyzed. CV at 24 months of age (0.68 ± 0.1, n = 10) was significantly larger than that of 2-3-month of age (0.34 ± 0.05, n = 18, P = 0.015), indicating that the regular pattern of spontaneous firing is disrupted in 24-month-old mice. These findings suggest prominent changes in firing properties of striatal cholinergic interneurons during aging.