Airway resistance and thoracic gas volume were measured in 21 normal infants aged 1 to 10 months using a whole body plethysmograph. Precautions were taken to eliminate temperature changes of inspired and expired gas. The mean value of airway resistance was 22·6 ± 8·9 (SD) cm H2O/l. per sec, and the mean thoracic gas volume in relation to body weight 34·9 ± 5·8 (SD) ml/kg. The reproducibility of the technique was assessed by making repeated measurements in 5 subjects, when the mean coefficient of variation of airway resistance was 11·8% and of thoracic gas volume 10·1%.