Abstract The hysteroscopic approach to the correction of congenital anomalies in utero may be helped by the temporary introduction of clear, electrically neutral fluids (liquids or gases) into the amniotic cavity. Accordingly, the authors determined the effects of three candidate fluids (helium, carbon dioxide, and water) on the oxygenation, acid-base balance, electrolyte concentrations, and hemodynamic function of nine sheep fetuses at 120 days' gestation (full term 145 days). Each fetus was exposed, in random order, to each of the three fluids, at an intraamniotic pressure of 15 mm Hg. The authors found that helium produced no changes in any of the measurements and was well tolerated by the fetus and the mother. Carbon dioxide decreased fetal arterial pH, and increased both arterial P co 2 and P o 2. Although the decrease in pH was explained entirely by the increase in P co 2, the increase in arterial P o 2 may have resulted from vasodilatation of the placental circulation or vasoconstriction of the fetal circulation with a redistribution of blood flow to the placenta. Water caused mild fetal hyponatremia and hypochloremia, without significant changes in acid-base balance or oxygenation. The results suggest that both helium and water may represent viable alternatives for amniotic fluid replacement during hysteroscopic surgery.