To study the effects of exogenous growth hormone (GH) administration to the mother on fetal growth, dwarf rats with an isolated GH deficiency were given daily s.c. injections of GH throughout pregnancy. Fetuses were heavier in GH-treated mothers (p < 0.001), and pups from GH-treated mothers continued to grow faster (p < 0.001) than pups from control mothers throughout the postnatal growth period through to weaning. In normal Wistar rats, administration of a potent antiserum to somatostatin to pregnant rats increased the mean birth weight of the offspring (p < 0.01). Administration of GH to the mothers also significantly increased birth weight (p < 0.05), but administration of antiserum to rat GH resulted in a significant retardation of both fetal and placental growth (p < 0.001). These data suggest that maternal GH status is significantly involved in the growth of the fetal rat.