Two lines of mice selected for reproductive longevity were compared with one unselected control line to examine longevity and lifetime performance such as number of parturitions during lifetime. Mice were pair mated at about 8 weeks of age and cohabited throughout life. Litter size was reduced to eight at birth in selected line 121 and unselected control line 221, but not in selected line 141. At generation 16, the average number of days from mating to the last parturition was 240.8, 243.0 and 134.6 for lines 121, 141 and 221, respectively. Selected lines 121 and 141 had 80% and 93%, respectively, greater number of parturitions during their lifetimes than the control line. Mean litter size at the early stage of reproduction was similar in the selected and control lines, but persistency of reproduction differed markedly. The average number of young born alive per breeding pair during a lifetime was 77.2, 82.8 and 45.9 for lines 121, 141 and 221, respectively. Data from the short-lived half of the mouse lines showed that mean lifespan was 17% longer in the selected lines 121 and 141 (377.5 and 436.5 days, respectively) than the control line 221 (346.8 days), and phenotypic and genetic correlations between reproductive longevity and lifespan were in the neighbourhood of 0.75 and 0.25, respectively. It was concluded that reproductive longevity was improved through selective breeding and lifespan increased as a correlated response to selection for increased reproductive longevity. The mouse lines derived from this study represent a unique mammalian model for studying various aspects of aging and longevity.