This experiment was performed to determine if the fetus can influence its placental metabolism. Fetal pigs were decapitated at 45 days of gestation and their placentas compared at 110 days to within litter controls. No histological or histochemical differences were observed. Glycogen levels were elevated in the fetal placenta of decapitated pigs by 107%. Fatty acid synthesis was increased in the fetal placenta of decapitated animals (12.9 +/- 1.5 vs. 3.4 +/- 0.7 nmol acetate units/100 mg/2 h). Fetal decapitation increased fetal placental fatty acid esterification by 43% and oxidation by 125%. The altered lipid metabolism of the fetal placenta was reflected in enzymes associated with substrate utilization (pyruvate kinase and lactate dehydrogenase). Maternal placenta lipogenesis was also altered by fetal manipulation. Leucine conversion to its alpha ketoacid was increased in the fetal placenta of the decapitated pigs (14.8 +/- 1.7 vs. 8.6 +/- 1.4 nmol/100 mg/2 h) indicating amino acid metabolism was affected by this treatment. The ability of an altered fetal endocrinology and metabolism to affect the placental metabolism supports the concept of a fetal influence on placental metabolism.