The effects on the conceptus of persistently decreased maternal plasma amino acid concentrations were studied in pregnant rats by the infusion of glucagon (0.21 mg/day) to the mother from day 14 to 20 of gestation with a subcutaneous, osmotically driven minipump. Controls received diluent. The experimental animals either had normal caloric intake and weight gain, or diminished caloric intake with no weight gain. Both experimental groups exhibited a decrease in plasma total amino acid concentration of approximately 50%. Maternal plasma glucose and insulin concentrations were unaffected except for slight decreases in the low weight gain group. At cesarean section on day 20, fetal weight was unaffected in the normal weight gain group, while the low weight gain animals exhibited intrauterine growth retardation. Fetal plasma glucose and insulin concentrations were unaffected. Despite the marked decrease in maternal plasma total amino acid concentration, fetal plasma total amino acid concentration was unaffected. Individual plasma amino acid concentrations in the normal weight gain mothers and fetuses revealed a spectrum of changes. Some maternal amino acids were decreased by more than 60% (α-aminobutyric acid, asparagine, threonine, glutamine, alanine) while others were unaffected (tyrosine, tryptophan, phenylalanine, histidine). In general, amino acids that were decreased in the mother exhibited no change or a lesser decrease in fetal plasma concentration, while those that were unaffected in the mothers showed increased fetal concentrations. Fetuses from the low weight gain mothers had plasma amino acid profiles that were similar to those of the normal weight gain mothers. We speculate that the preserved fetal plasma amino acid concentrations are due to enhanced placental transport of amino acid secondary to maternal substrate decrease.