To quantify changes in the pattern of the electroencephalogram (EEG) in relation to gestational age, a continuous two-channel record of the EEG with electrocardiogram (ECG) and respiration was made for the first five days after birth. 25 healthy babies with gestational ages of 26 to 42 weeks were studied. All were developmentally normal when assessed at 18 months. The EEG was divided into two distinct patterns; trace-alternant activity and continuous activity. The duration of trace-alternant activity and the mean duration of each epoch of trace-alternant activity decreased with increasing gestational age. Epochs of continuous activity increased in duration with increasing gestational age. Within epochs of trace-alternant activity the duration of each burst was independent of gestational age, but the mean inter-burst interval and the maximum voltage of slow-wave decreased with increasing gestational age. Analysis of variance showed all changes in relation to gestational age to be significant.