SummaryContinuous measurements of minute ventilation (&OV0312;I), oxygen consumption (&OV0312;O2), heart rate (HR), activity, and temperature were made in eleven low birth weight infants during the interval between feedings. Significant increases in &OV0312;I, &OV0312;O2, and HR were noted between quiet and active sleep. (&OV0312;I Active− &OV0312;I Quiet/&OV0312;I Quiet) × 100 = 18.4% &OV0312;O2 Active− &OV0312;O2 Quiet/&OV0312;O2 Quiet) × 100 = 10.1% and HRActive− HRQuiet/HRQuiet) × 100 = 6.4%. Significant differences were also noted within epochs of the same state of sleep: mean slope &OV0312;Iversustime in epoch (t) = −156 ml/kg·min/hr, &OV0312;O2versust. = 1.49 ml/kg·min/hr and HRversust = −15.0 beats/min/hr. Differences between successive epochs of the same state of sleep were also observed: &OV0312;I, +5.9 to 46.6%; &OV0312;O2, 4.7 to 24.6%; HR, 1.0 to 9.7%. These differences were related to the length of time after feeding. These data indicate that steady state conditions do not occur in growing low birth weight infants and that the design of studies of respiration and metabolism in these infants should include continuous assessment of the state of sleep or activity and time after feeding to ensure that experimental and control periods are truly comparable.SpeculationThe significant variability in minute ventilation, oxygen consumption, and heart rate which occurs spontaneously in low birth weight infants secondary to changes in activity and postprandial interval must be acknowledged in the design of clinical research. It is probable that these factors influence not only mean levels of metabolic and respiratory activity but also the sensitivity to experimental stimuli such as hypoxia, hypercarbia, and drugs.