Adults show distinctive patterns of slow-wave (delta) electroencephalogram (EEG) activity across each sleep cycle and across the night. We examined the ontogeny of slow-wave EEG patterning in infants. Twelve-hour overnight physiological recordings were obtained from 25 normal infants at 1 week and 1, 2, 3, 4 and 6 months of age. The EEG activity was band-pass filtered, leaving primarily activity ranging from 0.5 to 2.5 Hz (the delta frequency). Filtered EEG traces were full-wave rectified and integrated over 1-minute periods. Nighttime recordings were divided into four 3-hour segments, beginning at sleep onset, and the mean integrated delta activity during quiet sleep was determined for each segment of the night. In addition, patterns of delta activity across extended periods of quiet sleep (15 minutes or longer) were determined. Beginning at 2 months of age, integrated delta activity declined significantly over the night. Moreover, beginning at 3 months of age, delta activity increased significantly over individual periods of quiet sleep; in neonates up to 1 month of age, delta activity decreased significantly within epochs of quiet sleep. Beginning at 2-3 months of age, infants show patterns of delta activity similar to those found in adults.