Cerebral arterial and venous blood velocities were measured using pulsed Doppler ultrasound in 25 newborn infants requiring intermittent positive pressure ventilation (IPPV). The aim was to investigate whether these velocities altered in relation to the peak inflation pressure (PIP) used. Continuous recordings were made on the superior sagittal sinus and an intracranial artery both at the clinically prescribed PIP, and while this was altered in steps. A majority of the infants had, at some stage of their illness, variations in their venous and arterial velocities in phase with IPPV and related to the PIP used. These variations could be reduced by lowering the PIP. This was done in 5 infants whilst we were still able to maintain adequate ventilation.