Postnatal serotonin (5HT) innervation in the cerebral cortex of mice has been studied by 5HT immunohistochemistry. 5HT-like immunoreactive (5HT-LI) nerve fibers and terminals appeared to increase transiently, particularly in the somatosensory (Sm) cortex during early postnatal days. As pups grow, 5HT afferent inputs decreased rapidly to reach a similar pattern of distribution to that in adult animals. Since the transient increase was seen at a critical period (seventh postnatal day) for the differentiation of layer IV, it is suggested that increased 5HT concentrations might have an effect on thalamocortical inputs and/or cortical lamination of the developing brains.