Respiratory dead space, as commonly detd., includes the volume of the air passageways (anatomical dead space) and a hypothetical physiological dead space which accounts for the failure to utilize the volume of new air fully in gas exchange with the blood. It is suggested that the anatomical dead space be separately measured to give the rate of actual lung ventilation and that the efficiency of utilization be detd. by other means. The volume of anatomical dead space is obtained from the CO2 output per breath and the avg. CO2 concn. in the terminal fraction of normally exhaled air after completion of upper respiratory washout. This volume does not change with tidal volume at least, in passive respiration in a mechanical respirator. For 5 healthy young men the avg. anatomical dead space in 100 tests was 130 cc. [plus or minus] o = 30 cc. over a range of tidal volume from 400-2500 cc.