1. In four normal subjects and 51 patients with chronic bronchitis-emphysema, bronchial asthma and pulmonary fibrosis, the V D / V T ratio was measured by a helium washout technique (w V D / V T ) and by the conventional Bohr's method using the arterial CO 2 tension ( V D / V T ). 2. In 16 patients the w V D / V T ratio was compared with that calculated from Bohr's equation with the alveolar CO 2 tension measured by the rebreathing technique (A V D / V T ). 3. It was found that the values for the V D / V T ratio obtained by the helium washout technique were very close to those obtained by the conventional method using the Paco 2 or P ¯ A co 2 values.