Objective: Fluid shifts have been suggested to occur with fetal bypass. The degree or mechanisms behind these volume changes (or location) have not been defined. We characterized the preceding and correlated the findings to plasma vasopressin concentrations, the critical peptide of osmoregulation. Methods: Seventeen ovine fetuses (105-111 days' gestation) were started on bypass and followed 2 hours after bypass. Hemodynamics and volume replacements needed to maintain minimum reservoir volume during bypass and normal physiologic parameters after bypass were recorded. Serial blood samples were collected to assess gas exchange and vasopressin levels. Changes in total tissue water content were measured for several organs and the placenta. Plasma volume, fluid shifts, and osmolarity were calculated. Results: Hematocrit values decreased by 15 minutes of bypass to 28% from 33% and then increased to 34% by 120 minutes after bypass, corresponding to a decreased fetal plasma volume of 79 to 72 mL/kg by 120 minutes after bypass. The majority of volume shifts (approximately 100 mL/kg) occurred during bypass, but additional volume replacements were required after bypass to maintain normal hemodynamics, resulting in overall losses of 0.8 mL · kg-1 · min-1. Losses were not accounted for by placental or organ edema. Vasopressin levels increased dramatically with bypass (39-51.5 pg/mL) and were strongly predicted by increased fetal plasma volumes (R2 = 0.90), whereas osmolarity was not significantly associated with plasma volumes. Conclusion: Fetal bypass leads to significant fluid shifts that correlate strongly with increasing vasopressin levels (but not changes in osmolarity). The placenta is not the primary site of volume loss. Rehydration of the fetus is necessary after bypass.