A novel potentiometric, enantioselective membrane electrode based on graphite paste (graphite powder and paraffin oil) has been constructed. The graphite paste is impregnated with 2-hydroxy-3-trimethylammoniopropyl-beta-cyclodextrin (as chloride salt) solution, 10(-3) mol l(-1). The potentiometric, enantioselective membrane electrode can be used reliable for S-captopril assay as raw material and from Novocaptopril tablets, using a chronopotentiometry (zero current) technique, in the 10(-6)-10(-2) mol l(-1) concentration range (detection limit 2 x 10(-7) mol l(-1)), with an average recovery of 99.99% (RSD=0.05%). The enantioselectivity was determined over R-captopril and D-proline. The response characteristics of the enantioselective, potentiometric membrane electrode were determined also for R-captopril. It was shown that only L-proline is the main interfering compound. The surface of the electrode can be regenerated by simply polishing, obtaining fresh surface ready to be used in a new assay.