Abstract: The glassy carbon electrode was modified through electropolymerization with a film of L-cysteine in a phosphate buffer medium of pH 5.0 by cyclic voltammetry,and the electrochemical behavior of norepinephrine(NEP) at this modified electrode was studied.It was found that the electrochemical oxidation of NEP at the modified electrode was significantly catalyzed essentially due to the presence of L-cysteine.Conditions for determination of NEP by cyclic voltammetry with the modified electrode were studied and optimized.In the phosphate buffer of pH 7.0,interferences due to epinephrine and ascorbic acid were eliminated and NEP was determined by measuring the reductive peak current(Ipc).Linear relationship between Ipc and concentration of NEP was obtained in the range from 5.0×10-7 to 1.2×10-4mol·L-1and the detection limit(S/N=3) was found to be 2.0×10-8mol·L-1.The proposed method has been applied to the analysis of NEP injection samples,giving values of recovery in the range from 97.6% to 102.1%.