Arginine-vasopressin (AVP) and oxytocin are neuropeptides that are not only released as hormones into the peripheral circulation, but are also involved in central processes, e.g., in brain development. Earlier experiments suggested an inverse relationship between amniotic AVP and fetal growth. To see whether increased peptide levels reflect fetal growth retardation, and to determine cause and effect of this relationship, AVP and oxytocin content were determined in amniotic fluid of growth-retarded fetuses by radioimmunoassay. Growth retardation was established either by intraperitoneal administration of methylazoxymethanol to the mother, or by undernourishment of the mother. Elevated amniotic AVP levels were found in the methylazoxymethanol-treated and undernourished rats, partly concomitant with smaller amount of amniotic fluid. Amniotic AVP levels were inversely related to fetal body weight, while a similar trend was found for fetal brain weight. In addition, a positive correlation was found between fetal body weight and amniotic oxytocin in control rats.