The early and late changes in the right rat lung following single dose X-irradiation were studied using computerised axial tomography (CT) and noninvasive lung function tests. The biphasic response of the lung, i.e., the early pneumonitis phase, from six to ten weeks and later fibrosis starting at 18 weeks after irradiation, were both detectable by increased lung density. Whereas functional changes were prominent during the pneumonitis phase, these were only minor in the late phase.