The effect of intravenous aminophylline (10 mg/kg) on cerebral blood flow (CBF) and single flash visual evoked potential (VEP) was investigated in 16 preterm infants (range 25-34 weeks). All infants were weaned from mechanical ventilation and had normal brain ultrasonograms. CBF, PaO2, PaCO2, mean arterial blood pressure and VEP were recorded before, immediately after and 1 h after the administration of aminophylline. When intra-individual changes in PaCO2 were taken into account, mean CBF was 5.5% lower than the baseline value immediately after administration of aminophylline, but this was not statistically significant (P = 0.14). After 1 h, however, CBF had decreased by a mean of 13.8% (P = 0.0003). The cerebral function estimated from VEP was not influenced by aminophylline since the VEP configuration and the VEP parameters remained constant throughout the study. It is concluded that aminophylline reduces cerebral blood flow in stable, preterm infants and apparently without inducing adverse effects on cerebral function.