Alternative current polarography has been applied to indomethacin determination in serum, after dichloromethan extraction at pH 3.0. Solvent is evaporated and the residue submited to hydrolysis by NaOH. The indole derivative obtained is nitrosated by nitrous acid at pH 3,5. The indole-nitrosamine formed is measured by polarography at pH 1,3 and compared with an added quantity of the pure nitrosated product. The proposed method is selective and sensitive; it allows measurement of concentrations in serum as low as o,30 μg ml−1(0.83. 10−9moles ml−1).