The CSF plays an important role in removing transmitter amine metabolites from the brain and their measurement therein has frequently been used to study transmitter amine metabolism in the human brain during life. Much data, albeit largely qualitative, indicates that brain transmitter metabolism is reflected in the CSF. There is however remarkably little quantitative data available as human CSF and brain material are rarely concurrently available from the same patient. Conversely, until recently we have had much direct information on transmitter metabolism in the rat brain but CSF studies have been few, largely because of technical difficulties in measuring the metabolites. Therefore we have had little quantitative data on brain--CSF relationships for transmitter amine metabolism in any one species. Such evidence is important not only because it sheds light on the relevance of human CSF findings to brain chemistry but also because it may be used to study transmitter metabolism in the brain of the living rat and provides quantitative data on the role of the CSF in the removal of the metabolites from the brain. We will be reporting findings on these topics. These were obtained by the withdrawal of small volumes of cisternal CSF from conscious freely moving rats via an indwelling catheter and determining dopamine and 5-hydroxytryptamine metabolites by HPLC in both CSF and brain with and without administration of various drugs.