SummaryPlacental blood flow in the maternal side and transfer of [14C]-α-amino isobutyric acid (AIB) and [3H]methylglucose (MG) were measured in 22 pregnant guinea pigs at various gestational ages. The fetuses were divided in three groups according to their body weights: small, average, and large. Body weight was 85.25% of average values in the small fetuses and 114.12% in the large fetuses. Placental weight was 121.73% of average in the large fetuses and 84.42% in the small fetuses. Placental blood flow was 134.48% of average in large fetuses and 73.18% in small fetuses. AIB and MG transfers were significantly lower in the small fetuses (80.33% and 86.06%, respectively, of average values). In contrast, in large fetuses, AIB transfer was 123.43%, and MG transfer was 113.96% of average. Significant correlations were found between fetal and placental weight and placental blood flow and transfer of AIB and MG in the various groups. Placental transfer of AIB and placental blood flow were significantly correlated in the small (r= 0.59) and average weight fetuses (r= 0.37). In addition, the slope of the regression curve for AIB was significantly steeper in the small fetuses when compared with the slope of average and large fetuses. Placental blood flow and transfer of MG were significantly correlated only in the large fetuses (r= 0.48). In the small fetuses, the rate of AIB transfer was proportionally more reduced than that of MG transfer as the rate of placental blood flow decreased.The results demonstrate that spontaneous fetal growth retardation in the guinea pig is associated with a smaller placenta, a reduced placental blood flow, and a reduced transfer of AIB and MG. Inasmuch as in the small fetuses AIB transfer was proportionally more reduced than MG transfer, it is suggested that in addition to the reduced blood flow limited availability of certain essential ammo acids may be a cause of fetal growth retardation.SpeculationA reduced placental transfer of certain essential amino acids may further aggravate the fetal growth retardation associated with low placental blood flow.