Poor nutrition is a major cause of fetal growth restriction which increases neonatal morbidity and mortality, as well as the risk of adult onset diseases. The objective of the study was to determine the effect of maternal undernutrition on P-glycoprotein (P-gp) expression in the placenta and the brain of both the mother and the fetus. Maternal undernutrition in guinea pigs caused placental restriction, and thus decreased fetal weight. Pups in the maternal undernutrition (UN) group had fewer capillaries in the placenta and more capillaries in the brain of the fetus. Placental, maternal and fetal brain MDR1 mRNA expression was the same in the Control and UN groups. Maternal undernutrition resulted in a significant decrease in P-gp protein expression in the placenta and fetal brain, but not the maternal brain. These findings indicate that maternal undernutrition may impact on fetal exposure to drugs administered to the mother during pregnancy due to changes in placental transfer.