We investigated changes in the patterns of interaction between mother and their offspring and discussed on growth of offspring, using two strains of inbred mice (C57BL/10 and DBA/2), which are genetically distant. Interaction was observed by recording behavior of mice on 8 mm-video tapes at regular intervals. Postnatal growth and development were observed in the following two aspects: measurement of the total body, tail, hind foot and ear lengths and body weight; observation of changes in the development of external characters. The results obtained are as follows: 1. The asymptotic weight and lengths were always larger in C57BL/10 than DBA/2. The order in the occurrence of morphological changes differed between the two strains of inbred mice, and postnatal development was earlier in DBA/2 than C57BL /10. 2. Young stayed inside the natalnest until their eyes opened, and immediately after eye opening, they went out actively of their nest for the purpose of access to their mother. 3. Maternal behavior was gradually decreased with aging of offspring except for the brooding behavior of C57BL/10. 4. Feeding behavior of mothers was not apparently different between the two strains, or rather two patterns were observed within either strain, i.e. 1) increasing time spent for feeding and 2) decreasing time spent for feeding after young was 10 days of age. The lower feeding activity of mothers affected the weight gain and behavior of their young.