Ten healthy infants were examined by polygraph at 2, 6, 12 and 20 weeks of life during night sleep. Significant changes in the pattern of respiration and EEG in the course of quiet sleep have been described; at 2–20 weeks, the respiratory rate decreased in the course of quiet sleep; in the EEG the number of slow waves increased, whereas more rapid waves decreased at 6–20 weeks; the number of sleep spindles decreased; the amplitude of 2–4 csec waves decreased at 2–6 weeks, the amplitude of slower waves increased at 12–20 weeks. These changes probably represent a change in the functional state of the central nervous system in the course of quiet sleep. It seems they should be considered in the analysis of infant responses.