Pulsed Doppler ultrasound was used to evaluate the cerebral blood flow velocity (CBFV), in the middle and anterior cerebral arteries in 10 infants before and after the administration of aminophylline (7.5 mg/kg). Mean CBFV, heart rate, blood pressure, oxygen and carbon dioxide tensions were recorded before the loading dose, and then at 1, 2, 6 and 24 h after completion of the infusion. Mean CBFV in the middle and anterior cerebral arteries were 16.8 cm/s and 10.8 cm/s respectively prior to the infusion. There was a significant decrease (P < 0.05) in velocities in both arteries at 1 and 2h post drug therapy, which returned to base values by 6 h, and remained as such at 24 h. The heart rate increased (P < 0.05) after the infusion, while no consistent changes were observed in blood pressure or oxygen tension. Carbon dioxide levels were significantly reduced (P < 0.05) at 2 h. The reduction in CBFV, however, was more than would be expected on the basis of the change in carbon dioxide levels alone, suggesting that other factors may be exerting an influence. While no adverse effects were noted in the infants studied, it is suggested that aminophylline should be used judiciously in sick preterm infants at highest risk from ischaemic brain injury.