The correlation between airway resistance (RAW) measured in a plethysmograph and three respiratory resistance parameters measured by forced random noise was evaluated. Forced random noise resistance parameters were the average resistance between 5 and 9 Hz (R5-9), the average resistance between 20 and 24 Hz (R20-24), and the extrapolated resistance at 1 Hz (R1). The authors studied 22 healthy, nonsmoking subjects, 10 of whom had a history suggesting childhood asthma. Serial measurements were made after inhalation of aerosols containing increasing concentrations of methacholine to obtain increasing concentrations of methacholine to obtain data in various stages of bronchoconstriction. There was a broad range of responsiveness to methacholine; the ratio of the peak to base-line values of RAW ranged from 1.1 to 6.3. R1, R5-9, and R20-24 were related to RAW by linear regression analysis. R1, and R5-9 showed excellent correlation with RAW (r=0.93 and r=0.91, respectively); R20-24 showed much poorer correlation (r=0.62). The slopes of the regression equations for R1 and R5-9 were 1.56 and 0.51, respectively; thus neither provided an exact measure of RAW. The data appear to be consistent with a model in which most of the methacholine-induced increase in resistance occurred in the peripheral airways and only a small fraction in the central airway.