A method is described for the automatic sampling of CSF from conscious freely moving rats and the simultaneous determination of 5-HIAA and HVA by high-pressure liquid chromatography with electrochemical detection (HPLC-EC). Timed samples (30 microliters) were taken directly from a chronically implanted cannula in the cisterna magna of adult female rats by the HPLC autoinjector. The low volume of sample permitted withdrawal of repeated samples without cross-contamination. The cannula could be used the day after implantation and remained patent for at least two weeks. Treatment with probenecid (200 mg/kg i.p.) caused an increase of both metabolites in the CSF. Pargyline (50 mg/kg i.p.) affected both the 5-HT and dopaminergic systems, reducing their metabolite levels. alpha-Methyl-p-tyrosine (200 mg/kg i.p.) acted selectively on the catecholaminergic system lowering HVA levels. The method accurately measured amine metabolite changes in the CSF due to pharmacological manipulations.