Male and female Wistar rats (n = 140) age 1.5 months were maintained in either wheel-cage units or cage units for their entire life span. Voluntary wheel exercise significantly increased the mean longevity of both male and female rats compared with that of control rats. Between and within groups, growth duration was positively related to longevity, and growth rate was negatively related to longevity. These factors may explain differences in longevity between exercise and control groups and differences in longevity between male and female groups. These factors of growth, here defined in terms of body weight increment, may possibly account for many instances of group differences in longevity