The timing and metabolic basis for the rapid increase then cessation of placental growth in sheep and the accompanying changes in tissue cellularity were defined in the present study. Placental growth proceeded rapidly from day 40 of gestation to an apex at day 75-80 with no change is tissue dry matter content observed thereafter to day 100. These macroscopic growth patterns are similar to those observed previously, but present results define an earlier apex in placental mass. Absolute growth rate of the placenta reached a maximum near day 55, as derived from the Gompertz equation, concomitant with a period of maximum hyperplastic growth between days 50 and 60. A rapid increase in DNA synthesis and tissue mass per nuclei number from day 40-50 was followed by proliferative growth to day 70. Net cellular proliferation apparently ceased by day 80 as indicated by the apex in DNA content and the beginning of a static, low rate of DNA synthesis that continued to day 100. Patterns of change in the fractional rates of protein synthesis (Ks) and accretion (Kg) were similar suggesting that changes in Ks explain much of the change in Kg. This study, through its identification of key phases in the cellular growth of the ovine placenta, has laid the foundation for future research on mechanisms of placental growth regulation.