Electroanalytical techniques for the in vivo measurement of neurotransmitters in brain tissue have been applied especially to the catecholamines, which are easily oxidizable1–6. Measurements are, however, complicated by the presence of ascorbic acid (AA) in brain tissue1,3–6. Lane et al.7 have been able to circumvent this problem, at least in part, by the application of differential pulse voltametry (DPV) to a surface-modified platinum electrode, obtaining distinct oxidation current peaks in recordings from the rat neostriatum which are attributed to AA and to dopamine (DA), respectively, but which are also unstable. We have recently described a new type of electrode2,8, consisting of a pyrolytic carbon fibre 8 µm thick and 0.5 mm long. We now report that the DPV method used in conjunction with an electrochemical treatment of this electrode yields stable and reproducible peaks in which catecholamines and AA are resolved from each other. Moreover, pharmacological investigations suggest that the catecholamine peak measured in vivo in the rat neostriatum should be attributed to 3, 4-dihydroxy-phenylacetic acid (DOPAC), suggesting that our technique may be a useful means of following dopaminergic activity in vivo.