The effect of growth hormone (GH) on fetal growth was examined in the spontaneous dwarf rat (SDR) with the isolated GH deficiency. A 328-bp GH gene fragment containing the site of SDR mutation was amplified from fetal liver genomic DNA using PCR and digested with restriction enzyme Sau I to distinguish the fetal genotype. When heterozygote (dr/+) males and females were crossed, a significant reduction in body weight of dr/dr fetuses was observed on days 20 and 21 (92 and 89% that of +/+ fetuses, respectively). Body weight reduction was also noticed on day 21 in dr/dr fetuses (80%) in comparison with that of dr/+ fetuses crossed from dr/dr females and dr/+ males. Unexpectedly, the body weight of dr/+ fetuses was found to be significantly less than that of +/+ fetuses on days 19 (94%) and 20 (97%), which reached the level of +/+ fetuses by day 21 of gestation. The fetal tail length was not affected by the GH status. These results provide direct evidence for the involvement of pituitary GH in the regulation of fetal body weight during late gestation, but not in fetal skeletal growth.