The reduction in the effectiveness of the blood-brain and blood-spinal cord barriers, previously seen in rats at the height of the acute episode of experimental allergic encephalomyelitis, has now been measured at various stages in the development of the disease up to 60 days after inoculation with guinea pig spinal cord in complete Freund's adjuvants. The marker of extracellular space, radioactively labelled mannitol, only crosses the blood-central nervous system barriers very slowly by passive diffusion in normal rats. An abnormal penetration of this marker into the central nervous system began to develop during the second week after inoculation, appearing first in the lower spinal cord, where it also reaches the highest level during the acute phase of the attack. The leak begins before either the clinical signs become evident or cuffing is seen around blood vessels in stained sections. As the clinical signs are disappearing, from about 15 days onwards, the permeability of the barrier returns steadily to its normal low value, starting in the spinal cord, especially the caudal part. The timing of the reduction in the effectiveness of the blood-central nervous system barrier in relation to other clinical and histological changes suggests that it may play a part in the development of the lesion. The relation between the timing of these changes in EAE and that in the development of a new lesion in (exacerbation of) multiple sclerosis is discussed.