Errors in measurements arising from compression of gases in a mechanical ventilator may be corrected by calculation based on the compressible volume of the ventilator, or by physical separation of expired from compressed gases. Using a Frumin valve to effect this separation, the volume compressed in the Bear I ventilator was found to be 5.15 ml/mm Hg, and its compressible volume to be 3790 ml (range 3760 to 3806 ml). The ratio of uncorrected/corrected tidal volume (termed the correction factor) was 1.229 +/- 0.014. When VD/VT was corrected using this factor and compared with that obtained by calculating VD/VT using PECO2 measurements in expired air collected from the Frumin valve, the difference between the two was not significant. It is concluded that the use of the Frumin valve is a practical method for determining the true dead space-to-tidal volume ratios and that the application of the correction factor is also reliable.