Kr-81m gas has been used as a ventilation imaging agent. Continuous Kr-81m gas inspiration reflects the distribution of regional ventilation, but quantitative evaluation is difficult because of the short half-life of the gas.We obtained lung flow time activity curves during steady-state Kr-81m gas inhalation. Each respiratory cycle was defined between successive maxima of inspiration. Temporal Fourier analysis was applied to Kr-81m gas time activity curves to determine the pulmonary maximum expiratory (PER) and inspiratory rates (PIR) of the first harmonic of a single composite respiratory cycle. After forced inspiration of a bolus of Kr-81m gas, the time constant was calculated from the expiration time activity curve which was corrected for physical decay. These results were compared with a pulmonary function test and Xe-133 washout phase. Correlation between PER, PIR and % VC was noted. The time constant correlated negatively with % FEV1 and positively with mean transit time (MTT) of Xe-133 gas in the washout phase. Dynamic ventilation study appears to be a potentially useful noninvasive technique to assess local lung function.