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Summary: The rates of endochondral bone elongation during pregnancy and lactation in rats have been studied. The rate of growth at the distal femoral epiphyseal growth plate was measured using fluorescent bone markers. Endochondral growth rates were substantially increased in pregnant animals when compared with age-matched, nonmated controls. There were also increases in growth plate thickness, hypertrophic cell lacunar height, and the calculated rate of cell production during pregnancy. At parturition, this growth trend was reversed and during lactation there were significant decreases in endochondral growth rates. There were also corresponding decreases in growth plate thickness, hypertrophic cell lacunar height, and the calculated rate of cartilage cell production. These results indicate that significant changes occur in maternal endochondral growth rates during the reproductive cycle in rats.