We have studied haemodilution and cardiovascular responses to i.v. hydration with either 3% dextran 70 (Dx70) or Ringer's acetate using a non-invasive cardiac output monitor (BoMed NCCOM3-R7) in 40 healthy parturients undergoing Caesarean section under extradural anaesthesia. Haemodilution was more pronounced, and central venous pressure, mean arterial pressure and cardiac index maintained at greater values, after treatment with Dx70. Colloid osmotic pressures (COP) decreased by 1.7 mm Hg after loading with 3% Dx70, and by 5.6 mm Hg in mothers treated with Ringer's acetate (P less than 0.001). The transthoracic fluid index decreased more after hydration with Ringer's acetate (P less than 0.001), indicating an increase in lung water. In spite of these maternal changes, there were no differences in neonatal bioimpedance or values of haemoglobin, PCV, albumin and COP in umbilical cord blood, and only one case of respiratory distress. We conclude that colloids may be preferable to crystalloids for circulatory preload for extradural Caesarean section, as greater haemodynamic stability was maintained and increases in lung water avoided.