Infantile behavior development was investigated in the senescence accelerated mouse (SAM)-R/1 and P/8. No differences were found in the physical growth (body weight and time for morphodifferentiation). It was found that there were four stages in the behavioral development when measured by reflexes, locomotive activities, grooming, orienting, tropistic creeping and bar-holding ability. In the fourth stage, athletic exercise-ability in the SAM-P/8 was less than that in the SAM-R/1. This difference was caused by changes in maternal behavior (learning and memory impairments), confirming the importance of the mother. These results indicated that behavioral characteristics in SAM-P/8 begin in infancy.