Quantitative nuclear magnetic resonance imaging techniques were used to study the development of astrocytic gliosis following a cortical freezing injury in 4 cats, and the findings compared with the histological and ultrastructural appearances of the affected white matter. In the first week after lesioning, the acutely oedematous white matter was apparent as a region of signal abnormality with very prolonged T1' and T2' relaxation times. Following resolution of this oedema, the images remained normal for a period of approximately three months. After this latent period they became abnormal once again, and the abnormal signal was most evident in images emphasizing differences between tissues in T1' and proton density, but not T2'. The light and electron microscopic appearances of the corresponding tissue were of astrocytic gliosis in each animal. Measurement of the relaxation times, T1' and T2', of the abnormal regions revealed an increase in T1' without a corresponding change in T2'. The T2' magnetization decay characteristics were consistently monoexponential, in contrast to the clearly biexponential T2 decay which has previously been demonstrated in the presence of acute vasogenic oedema. These findings suggest that the pattern of change of the relaxation times and T2 magnetization decay might provide a means of distinguishing between lesions in multiple sclerosis which are predominantly gliotic, and those containing significant amounts of oedema.