The performance of an infra-red analyser (Beckman LB-2) in sampling alveolar gas at ventilatory frequencies and volumes occuring in the human neonate has been examined. The optimum sampling flow rate was found to be 300 ml/min; at this flow the 90% response time of the analyser-catheter system was 140 ms. Correlation of estimated alveolar PCO2 and arterial PCO2 was performed in the rabbit which has a ventilatory rate and tidal volume similar to those of the human neonate. There was a good correlation between maximum end-expired PCO2 (PE'CO2) and arterial PCO2 over a wide range of ventilatory rates (10-100 b.p.m.); maximum PE'CO2 was a better index than mean PE'CO2 or alveolar plateau PE'CO2.