The transient hyperaemic response (THR) of blood flow velocity in the middle cerebral artery (vmca), measured by transcranial Doppler ultrasonography (TCD), can be used to assess cerebral autoregulation. We have studied the effects of propofol administered by target-controlled infusion on vmca, THR and carbon dioxide reactivity. We studied 20 healthy adult patients undergoing elective surgery. A standardized anaesthetic comprising alfentanil 10 micrograms kg-1, propofol via a target-controlled infusor and vecuronium 0.1 mg kg-1 was used in both parts of the study. In the first part, THR tests were performed on 10 subjects while awake and then at an 'induction' target concentration of propofol (the target at which consciousness was lost, mean 6.7 (SD 1.1) micrograms ml-1). In the carbon dioxide study, reactivity was tested in 10 patients while awake and at the 'induction' target concentration of propofol by altering the end-tidal carbon dioxide partial pressure by 1 kPa either side of baseline. Propofol caused a significant decrease in vmca but indices of autoregulation, THR ratio and strength of autoregulation increased significantly. Propofol had no effect on carbon dioxide reactivity. These results suggest that propofol may have a beneficial effect on cerebral haemodynamics.