THE SOLUBILITY OF HALOTHANE in blood is influenced by several factors such as lipid) protein and haemoglobin concentrations, 2 and haematocrit. During cardiopulmonary bypass two other factors may alter halothane solubility; these are hypotbermia '*'s's and haemodilution.V The purpose of this study was to determine the blood concentration and the. blood/gas partition coefficient of halothane during cardiopulmonary bypass with haemodilution in conditions of hypothermia and normothermia. MATERIALS AND METHODThe study included six patients undergoing open-heart surgery for the correction of congenital anomalies with mild hypothermia (30 ~ C) and six patients undergoing open-heart surgery for acquired heart diseases under normothermia (36*-37 ~ C).A previously calibrated Fluotec Mark 1I vapourizer was inserted in line along the tubing delivering oxygen to a bubble oxygenator at a rate between 3 and 5 L/min in the paediatric hypothermic cases and between 6 and 8 L/min in the adult normothermic cases. Blood flow through the pump was maintained between 70 and 90 ml-kg -I-rain -t in the former and 50 to 70 ml. kg -t-rain -I in the latter. Arterial blood gases and acid-base balance were kept within normal limits throughout the study. In both groups, two different concentrations of halothane were studied during cardiopulmonary bypass; first, a low concentration consisting of 0.6 per cent was given; then the high concentration administered