Right (RVFW) and left (LVFW) ventricular free wall cardiac myocytes were collected from 25 fetal sheep aged 77-146 days gestation (term ϭ 150 days gestation), six salineinfused catheterized fetal sheep (129 GD), and five lambs to measure gestational changes in uni-and binucleated cardiac myocyte numbers and cell volumes by confocal microscopy. At 77 days gestation, 2% of the myocytes were binucleated, which increased to 50% at 135 days gestation and 90% at 4 -6 weeks after birth. RVFW uni-and binucleated myocytes were larger than those in the LVFW, and cell volumes of RVFW uni-and binucleated and LVFW binucleated myocytes (but not LVFW uninucleated myocytes) increased with gestation. Before birth, the approximate number of myocytes was greater in the LVFW than in the RVFW (P Ͻ 0.001). Before 110 GD, cardiac growth appeared to be due to myocyte hyperplasia, as approximate myocyte numbers and VFW weight increased at the same rate. After 110 days gestation, the approximate myocyte number/g VFW weight decreased, which suggests that myocyte hypertrophy, as well as hyperplasia, was occurring in association with the appearance of a greater proportion of binucleated cells after that time. By 4 -6 weeks of age, there was marked hypertrophy of myocytes and an apparent reduction in myocyte number. Anat Rec Part A 274A: 952-961, 2003.