Human butyrylcholinesterase (BuChE) is being developed as a new therapeutic for protection against the toxicity of organophosphorus agents and cocaine. The purified BuChE consists predominantly of 340 kDa tetramers and contains less than 5% monomers and dimers. Our goal was to determine whether BuChE crosses the blood-cerebrospinal fluid (CSF) barrier. Rats were injected intraperitoneally with 1mg of purified human BuChE. Plasma BuChE activity increased nearly 400-fold, while BuChE activity in the CSF increased three-fold. Sucrose density centrifugation showed that the human BuChE molecule in the rat CSF was a tetramer. Immunoprecipitation confirmed the identity of the CSF BuChE as human BuChE. The lower amount of human BuChE in the CSF (0.04%) than of smaller proteins (0.1-1%), with respect to their levels in plasma, supports the idea that passage through the blood-CSF barrier depends on molecular size. BuChE in the CSF could serve to protect the brain from the neurotoxicity of organophosphorus pesticides and cocaine.