Long-term selection for body weight, which affecting changes in life span and time of puberty, has resulted different reproductive strategy in mice divergently selected for body weight at the 21st day of age. Heavy mice (C) are typically rapidly maturing animals, with a shorter but more intense reproductive period. Light mice (L) are more long-lived so that mature later than C mice, Due to the longer period of reproduction L mice are capable of more litters, which are also less numerous. Moreover, long-term selection may changes of mice. L mice showed lower activity in the open field test. Light mice are more susceptible to stress, and they have higher excitability, than heavy mice.