Serotonin (5-HT) is an important signal molecule not only for neurones but also for a variety of other cell types. Targeting the brain endothelium, the constitutive element of the blood-brain barrier (BBB), it elicits permeability changes. Using the selective 5-HT uptake inhibitor [3H]paroxetine we demonstrated the presence of a 5-HT transporter-like protein at the BBB. The binding capacities (Bmax) at the BBB (382 fmol mg-1) and on caudate nucleus membranes (392 fmol mg-1) were similar. However, the binding affinities differed by a factor of 5 (membranes: Kd = 0.10 nM, BBB: 0.47 nM). The affinities of various specific uptake inhibitors were also 2- to 13-fold lower in the microvessel preparation. It is suggested that the 5-HT transporter(s) in the brain and microvessels are different or differently regulated proteins.