Alkaline ionized water (AKW) produced by electrolysis was given to gestational and lactational rats, and its effect on dams, growth of fetuses and offsprings were investigated. The results showed that the intake of food and water in dams increased significantly when AKW was given from the latter half of the gestation period and from the former half of the lactation period. Body weight of the offsprings in the test group, both males and females, increased significantly from the latter half of the lactation period. During the lactation period and after weaning, the offsprings in the test group showed significantly hastened appearance of abdominal hair, eruption of upper incisors, opening of eyelids and other postnatal morphological developments both in males and females, as well as earlier separation of auricle and descent of testes in males compared with the control was noted. As mentioned above, it was suggested from the observations conducted that the AKW has substantial biological effects on postnatal growth, since intake of food and water and body weight of the offsprings increased and postnatal morphological development was also accelerated.