Transcranial pulsed Doppler ultrasound and spectral analysis were used to monitor blood velocities in the middle cerebral artery of nineteen patients (mean age 61 +/- 9 years) during carotid endarterectomy. A Javid shunt was used in all patients. The intensity weighted mean Doppler frequency for each spectral sweep (at 5 ms intervals) was time-averaged over the cardiac cycle to obtain a mean value for blood velocity in the middle cerebral artery. The range of such values found in the 19 patients was: 12-38 cm s-1 after anaesthesia (baseline); 12-69 cm s-1 during diathermy; 0-30 cm s-1 during carotid clamping; 16-32 cm s-1 during shunting and 18-60 cm s-1 in the recovery room. The average change in middle cerebral artery blood velocity from baseline values showed significant increases during diathermy (P less than 0.005), shunting (P less than 0.05) and in the recovery room (P less than 0.005). Clamping of the internal carotid artery showed a significant decrease in middle cerebral artery blood velocities of all patients (P less than 0.005), three of whom showed no flow in the middle cerebral artery during clamping. Abnormally high amplitude Doppler signals at the commencement of shunting were detected in 17 of the 19 patients. Such Doppler signals are consistent with turbulent blood flow or the introduction of micro-air bubbles by the shunt. Backbleeding in the internal carotid artery before insertion of the shunt was associated with diminished flow in the ipsilateral middle cerebral artery of ten patients, oscillatory forward/reverse flow in three patients and cessation of flow in the remaining six patients.