This study was to investigate the influences of habitual mouth breathing on memory and learning ability during the growth period. At age 5 weeks, the experimental rats were subjected to surgery to close completely one side of the nasal cavity. An 8-arm radial maze was used to evaluate memory and learning ability at age 7, 11, and 15 weeks. Moreover, the brain was extracted at age 7, 11, and 15 weeks, and subjected to histomorphometric examination for the distribution and number of pyramidal cells in the hippocampal CA1 and CA3 regions after Nissl staining. The trial time to accomplish each task was significantly longer in the experimental group than in the control group throughout the experimental period. The number of pyramidal cells was significantly less in the experimental rats than in controls in both the CA1 and CA3 regions for the entire experimental period. Thus, the functional deterioration of the respiratory system during the growth phase exerts a substantial effect on the growth and development of the central nervous system.