Plethysmographic rhinomanometric resistance measurements of combined and separate nasal cavities were made at a transnasal differential pressure of 100 Pa. The coefficients of variation over time of 40 consecutive total resistance values obtained at 1 min intervals from untreated noses of five healthy subjects averaged 11.0% measured directly and 11.8% calculated by application of Ohm's Law for parallel resistors. Measurements at 5 min intervals between sides increased variation of calculated total resistances markedly. The coefficients decreased to 4.7% and 5.1% respectively when the noses were decongested and by contrast with untreated noses these resistances varied independently from each other. Facial masking increased the coefficient of variation of resistance in the decongested nose (p less than 0.001) to as much as 11.0% and the magnitude of averaged resistances was moderately increased also (p less than 0.035). Measured values plotted against calculated values for total nasal resistance of 45 consecutive patients produced a regression differing insignificantly (p = 0.94) from the line of identity in the decongested nose but diverging from it (intercept 0.03 Pa/cm3/sec and slope 0.83, p less than 0.03) when the nose was untreated. Resistive variations associated with mucovascular instability and with use of a face-mask contribute substantially to differences between the results of anterior and posterior rhinomanometric assessments of total nasal resistance.