The lamprey choroid plexus was studied by electron microscopic techniques and the composition of cerebrospinal fluid and plasma compared as part of a characterization of the lamprey blood-brain barrier. It was shown that the ultrastructure of the lamprey choroid plexus is very similar to that of the mammalian plexus. A blood-cerebrospinal fluid (CSF) barrier to horseradish peroxidase (molecular weight 40,000) and microperoxidase (molecular weight 2000) was localized to apical tight junctions between the choroidal epithelial cells. Pinocytic uptake of the tracers took place particularly at the apical surface of the epithelium (after intravenous and intraventricular administration). Absorbed tracer-molecules were found in vacuoles presumably belonging to the well-developed lysosomal apparatus of the epithelial cells. Extended Golgi-complexes, dense bodies and some multivesicular bodies were reactive for acid phosphatase activity. Measurements of protein, potassium and sodium in plasma and CSF revealed the same concentration differences previously observed in higher vertebrates. The present study lends further support to the view that the lamprey blood-brain barrier is similar to that of higher vertebrates.