White-footed mice Peromyscus leucopus reared in our laboratory from wild-caught parents lived at least to 5.5 years. Once past weaning, over 80% of the mice lived at least until two years. Females maintained a constant weight whereas the weight of males increased until four years and decreased thereafter. At four-years of age both sexes bred successfully with mates of the same age, but had lower success with younger mice. Some males and females reproduced at five years of age.