In a longitudinal study of 217 infants delivering at < 37 completed weeks gestation, Doppler flow velocity waveforms were obtained and resistance index (RI) values calculated from the middle (MCA) and anterior (ACA) cerebral arteries during the first 10 days of life. One hundred thirty-seven of these infants were non-acidotic at delivery and during the early neonatal period, and had normal cerebral ultrasound scans throughout the study period. These infants formed the reference group. In three gestational subgroups considered (< or = 32 weeks, 33-34 weeks, > or = 35 weeks) from the reference group, the median RI for both the ACA and MCA was noted to fall significantly during the first 12 h of life (P < 0.01 for all groups). For infants delivering at > or = 33 weeks gestation, both MCA and ACA RI values reached a steady state with no significant change in the median value for the remainder of the study period. For infants delivering at < or = 32 weeks, there was a further significant fall in both the MCA and ACA RI between 12 and 24 h of life (P < 0.05), after which a steady state value was reached. During the first 12 h of life the RI for both vessels was significantly higher in infants delivering at < or = 32 weeks compared to the more mature infants (P < 0.01), but for the remainder of the study period, there were no significant differences in RI values between the gestational subgroups.(ABSTRACT TRUNCATED AT 250 WORDS)