SUMMARY Blood pressure changes as a function of the rates of blood withdrawal and infusion were determined during exchange blood transfusion in 2 preterm newborn infants. 10 ml blood withdrawal and infusion completed within 3 minutes resulted in reversible changes in aortic pressure, whereas the same procedure completed within 45 to 60 seconds resulted in a progressive fall in the systolic pressure and in narrowing of the pulse pressure. In one of the cases this was associated with respiratory arrest. An exchange rate of 5 ml/kg per 3 minutes is recommended.At present there is no rational recommendation as to the rate at which exchange transfusion should be performed. Our observations of blood pressure (BP) during slow and fast exchange transfusion in 2 preterm infants are reported to illustrate that significant alterations in BP are associated with exchange transfusion and that these changes are partly rate dependent.Case reports Case 1. A male, weighing 2270 g, of 36 weeks' gestation who had haemolytic disease of the newborn due to Rh incompatibility. Labour and delivery were uncomplicated and Apgar score at one minute was 8. Physical findings at birth included jaundice, hepatomegaly, and splenomegaly.Cord blood haematocrit was 37%, serum total bilirubin concentration was 6-3 mg/100 ml (108 ,umol/l) of which 1I0 mg/100 ml (17'1 ,umol/l) was direct, blood group was AB, he was Rh positive and the direct Coombs's test was positive. Jaundice and pallor increased and at 6 hours of age serum total bilirubin concentration had risen to 12 3 mg/100 ml (210 jsmol/l) (direct bilirubin 0*8 mg/100 ml; 13-7 iLmol/l) and the venous haematocrit had fallen to 20 -5 %. The concentrations of serum protein and calcium were 39 g/l and 8 * 8 mg/100 ml (2 -2 mmol/l) respectively. Mild respiratory distress was noted and a no. 5 French Argyle umbilical artery catheter was inserted to the level of the third lumbar vertebra to