• Cerebral blood flow decreases substantially in the adult after a single bolus injection of aminophylline. To determine if the cerebral circulation of the low-birth-weight infant behaves in a similar manner, we measured cerebral blood flow velocity (CBFV) in the anterior cerebral arteries of nine infants treated with an intravenous bolus dose of 5 mg/kg of body weight of aminophylline. A reduction in CBFV of 21% and 17% was observed at 60 and 120 minutes following the aminophylline administration. There was a concurrent substantial reduction in Pco2 from 45±7 to 39±7 mm Hg before and 120 minutes after the aminophylline administration, respectively. The reduction in CBFV may be the result of a reduction in Pco2 resulting from aminophylline treatment.