Pulmonary parenchymal tissue volume and capillary blood flow are measured with a rebreathing technique utilizing absorption of acetylene. Since stability of capillary blood flow is critical for measurement of tissue volume, special rebreathing techniques are being devised. Utilizing these techniqes, tissue volume will be evaluated in conditions of lung damage in dogs and eventually in acutely ill humans. Additionally, large numbers of ambulatory subjects will be studied to establish the effects of age, sex, height, and cardiopulmonary disease upon normal values.