SUMMARY The effect of random perturbations on the data used to calculate dead space/tidal volume ratio (Vd/Vt ratio) and percentage physiological shunt (Qs/Qt ratio) is examined. With a coefficient of variation of 1%, such random errors cause a true calculated Vd/Vt ratio of about 40% to lie within the range 38% to 43% (mean 40.5%, SD 0.8%), and a calculated Qs/Qt ratio to vary around a true value of about 20% from 10% to 28% (mean 20.4%, SD 1.5%). With a 2% coefficient of variation the corresponding ranges are 36% to 44% (mean 40.4%, SD 1.5%) for the Vd/Vt ratio, and 0% to 35% (mean 20.6%, SD 6.1%) for the calculated Qs/Qt ratio. Errors in the calculated Qs/Qt ratio may be minimized by deriving the arterial and mixed-venous oxygen contents, rather than measuring them directly. Errors having a 2% coefficient of variation then cause the calculated physiological shunt to lie between 16% and 24% (mean 20.1%, SD 1.4%).