Endopolyploidy is the generation of polyploid cells by DNA replication without subsequent cell division and is correlated with hypertrophic growth or growth via cell size. Thus, selection that alters growth may also change onset and frequency of endopolyploidy as a correlated response. We search for endopolyploidy in the liver in response to age-specific restricted index selection for the rate of development. Polyploidy changes over ontogeny are described in five mouse lines: two selected for divergence in early growth (0-10 days of age), two selected for divergence in late growth (28-56 days of age), and one randombred control. Polyploid cell frequency within each line increased as ontogeny continued, as expected from previous research. However, selection for altered growth clearly plays a role in the frequency and onset of polyploid cells. Lines selected for divergence in early growth have polyploidy differences after weaning that are not seen in adult mice. However, lines selected for divergence in late growth are divergent in frequency of polyploid cells, starting near sexual maturity and continuing into adulthood.