Labeled phenylacetic acid (14C-PAA) penetrates blood-cerebrospinal fluid (CSF) barrier in cats only under a high gradient of concentration; under these conditions the administration of large doses of either PAA or probenecid decreases the plasma:CSF 14C-PAA ratio. 14C-PAA is quickly removed from the CSF and blood and is excreted into urine; this excretion is blocked by probenecid. It appears, therefore, that PAA crosses the blood-CSF barrier with difficulty and that is secreted by the kidney.