In the rabbit it was found that probenecid inhibited the passage of 5-HIAA from the CSF to the blood, but was more active when given in fractionated doses. This confirmed that an active transport system for the 5-HIAA elimination across the CSF-blood barrier existed in the rabbit. Such a transport system did not appear to be present at the nervous tissue-CSF barrier since there was a slight in crease of the brain 5-HIAA under the probenecid action.