The category of striatal complex contains caudate nucleus, putamen, nucleus accumbens septi, and olfactory tubercle. The striatal complex is composed of two compartments, dorsal and ventral striatum. In the striatum, cholinergic neuron is known as one of the most important intrinsic neurons, but there were little morphological reports about the early postnatal expression of mouse striatal cholinergic neurons. So, we planned to investigate the distribution of mouse striatal cholinergic neurons in their early postnatal period by the immunohistochemistry. We used ICR mouse as the experimental animals and divided them into 5 groups according to their postnatal age : 3-day, 1-week, 2-week, 4-week, and 6-week. Immunohistochemistry was done with anti-choline acetyl transferase antibody (chemicon). The results were as follow. 1 The striatal cholinergic neurons are already detected in the 3-day group, but the intensity was weak and the expression rate was extremely low. In the caudoputamen, the cholinergic expression rate was increased significantly between 3-day and 2-week. And in the nucleus accumbens septi, it was increased significantly between 1-week and 2-week. 2. The cholinergic expression rates of the adult mouse striatum were similar in both compartments. But, the difference of maturational time was noted. In the dorsal striatum, the cholinergic expression rate was increased significantly in the first postnatal week, but in the ventral striatum, it was approached to the adult level only after second postnatal week. In conclusion, the cholinergic expression rate in the mouse striatum was significantly increased after birth. And it was approached nearly to the adult level after 2-week of postnatal age. But, according to the compartments or rostrocaudal subdivisions, the difference of maturational time was noted.