Cerebral Function Monitor (CFM) recordings were performed on 10 term and 19 preterm healthy infants. Term infants were monitored once, while preterm infants were followed serially. Forty-six recordings were made on 7, 14, 16 and 9 occasions in the age groups 30-31, 32-33, 34-35 and 36-37 weeks. All infants were examined clinically at 18 months of age and found healthy. By drawing weighted lines derived from the lower and upper limits of the CFM traces, mean values of minimum and maximum cerebral activity were calculated for the different age groups. In the term infants different CFM traces were identified corresponding to quiet sleep and active sleep. In the preterm infants a similar cyclic variability of the CFM trace was noted. A gradual increase in the minimum cerebral activity was found with increasing gestational age, resulting in a gradual narrowing of the trace.