A system is described for measuring carbon dioxide output and alveolar carbon dioxide concentration in the neonate and for deriving the volume of alveolar ventilation. Air is drawn through a soft rubber face mask applied loosely to the face so that all expired gas is entrained. A fraction of the flow is sampled by an infra-red analyser, the signal from which is integrated to provide a measure of carbon dioxide output. Alveolar carbon dioxide concentration is measured by continuous sampling of expired gas through a nasal catheter. The response time of the catheter-analyser system is 140 ms. In 20 healthy 1-day-old neonates the mean carbon dioxide excretion was 5.86 ml/min. kg-1 (STPD) and the mean alveolar PCO2 was 4.85 kPa. From these measurements mean alveolar ventilation 147 ml/min. kg-1 (BTPS) was calculated. These results are similar to those obtained by other methods.