The influence of perivascular sensory afferent nerve fibres upon the permeability of the blood-brain barrier was examined in halothane-anaesthetised rats. Blood-brain barrier permeability was assessed by the rate of transfer of the small neutral amino acid, [14C]alpha-aminoisobutyric acid, from blood to brain and by gross extravasation of Evans blue albumin. In every brain area examined, the acute administration of capsaicin (0.15-5 mumol/kg, i.v.) failed to alter significantly the passage into the brain of the small neutral amino acid tracer or to effect dye extravasation. Capsaicin, at concentrations which cause the release of vasoactive neuropeptides from nerve endings and increase vascular permeability in peripheral tissues, does not increase the permeability of the blood-brain barrier.