Although inadequate plasma volume expansion (PVE) and decreased fetal weight have been shown to occur following maternal malnutrition in rats, whether this association is one of cause and effect is unknown. By artificially expanding blood volume in malnourished animals on the 14th or 17th day of gestation with a single IV injection of whole rat blood we have been able to examine this relationship. Results demonstrate that when pregnant rats are restricted to a 5.5% protein diet throughout pregnancy but injected with 5 ml of virgin rat blood on day 14, plasma volume expands to levels seen in nonrestricted animals and the pups are of normal weight at term. By contrast if the PVE is performed at 17 days gestation growth failure will occur. Thus the growth failure attendant to maternal protein restriction can be prevented by artificially expanding blood volume early in pregnancy. These data suggest that a causal relationship exists between lack of maternal plasma volume expansion and fetal growth failure in dams subjected to protein malnutrition.