5-Hydroxyindole acetic acid and homovanillic acid in cerebrospinal fluid after intrathecal and intravenous administration of probenecid to normal and hydrocephalic dogs.

: This study has shown that intrathecal instillation of probenecid gives a lower elevation of the acid metabolites than intravenous administration of the drug, providing the concentration of the drug in the cerebrospinal fluid is the same. Furthermore, we have reason to believe that it is possible to block almost completely the active out transport of 5-hydroxyindole acetic acid from the cerebrospinal fluid by high doses of probenecid given intravenously or intrathecally. This does not seem to be the case with homovanillic acid.