Dynamic changes in motor abilities and motivated behaviors occur during the juvenile and adolescent periods. The striatum is a subcortical nucleus critical to action selection, motor learning and reward processing. Its tonically active cholinergic interneuron (ChI) is an integral regulator of the synaptic activity of other striatal neurons, as well as afferent axonal projections of midbrain dopamine neurons; however, little is known about its development. Here, we report that ChI spontaneous activity increases during postnatal development of male and female mice, concomitant with a decreased afterhyperpolarization. We characterized the postnatal development of four currents that contribute to the spontaneous firing rate of ChIs, including ISK, IA, Ih and INaP. We demonstrated that the developmental increase in INaP drives increased ChI firing rates during the postnatal period and can be reversed by the INaP inhibitor, ranolazine. We next addressed whether immature cholinergic signaling may lead to function...