A radionuclide 133Xe clearance technique has been used to assess changes in the regional gas exchange capacity of the lung after the irradiation of part of the lung of pigs with fractionated doses 250 kV X rays. Changes in the gas-exchange capacity of an irradiated volume of lung were compared with a similar volume of unirradiated lung in each animal. The results from the lung function tests were converted into quantal data on the basis of the percentage of lung function tests in animals showing a greater than or equal to 15% impairment of function in the irradiated lung compared with contralateral unirradiated lung in the same animal. From those quantal data, ED50 values (+/- SE) were obtained for each fractionation schedule. These ED50 values were used to determine time-dose-fractionation relationships for both early and late radiation-induced damage to the lung. There was apparently no recovery phase between the early and late phases of damage to the lung. In general, ED50 values for the late effect were slightly lower than those for the early effect, but this only proved significant when measurements obtained at 4 weeks after treatment were included. Varying the period for the assessment of early and late damage did not have a major effect on the results but did suggest that late damage was established in pigs by 39 weeks after irradiation. The linear-quadratic model did not provide a satisfactory fit to the fractionation results for pig lung. However, a modified nominal standard dose equation did provide a better description of the findings. A value of 0.44 +/- 0.06 was obtained for N exponent in the equation for both early and late lung damage when the single-dose data points were included in the analysis. The value of the N exponent was increased to 0.52 +/- 0.08 when the single-dose data were excluded. This increase was not significant.