In normal brain, the blood-brain barrier (BBB) is highly impermeable to K+ cations, their transport being controlled by ATPases situated in the endothelial cell membranes. 82Rb+ is a positron-emitting analogue of K+ with a half-life of 75 s. Using a steady-state model and positron emission tomography, quantitative extraction data for 82Rb+ transport across the BBB have been obtained both in normal human subjects and in a variety of conditions of cerebral pathology. A mean cerebral Rb extraction of 2.1% was found for normal subjects, corresponding to a mean value of 1.1 x 10(-6) cm s-1 for 82Rb+ cation permeability across the BBB. No increase in cerebral Rb extraction was observed for patients with diffusely raised intracranial pressure secondary to obstructive hydrocephalus and benign intracranial hypertension, or for patients with multiple sclerosis or cerebral systemic lupus erythematosus. Cerebral tumours that were enhanced on computed tomography scanning showed a significant increase in local Rb uptake. No correlation between tumour size, or grade of glioma, and tumour Rb extraction was found. Nonenhancing tumours showed no increase in local Rb extraction, and regions of perifocal tumour oedema also had Rb extraction values in the normal range. It is concluded that increased Rb extraction occurs only where tight junction integrity in the BBB breaks down locally, that is, in the microcirculation of enhancing tumours but not in that of perifocal regions of tumour oedema or nonenhancing tumours.