The determination of acetycholinesterase (AChE) was carried out based on the chemisorption/reductive desorption of thiocholine on a silver electrode surface. Thiocholine, which was produced through the hydrolysis of acetylthiocholine with AChE, chemisorbed on the silver surface during an incubation time; then the silver electrode was transferred into an alkaline solution and the adsorbent was electrochemically desorbed. The increase in the enzymatic activity resulted in the increase of the number of the thiocholine molecules chemisorbed and, accordingly, that of the cathodic current response for the electrochemical desorption. The thiol molecules were accumulated on the electrode surface through the chemisorption, which resulted in an enhanced sensitivity for measuring the enzyme activity. The detection limit for AChE was 0.01 Ul(-1), i.e., a hundred times lower than that obtained by a conventional amperometric sensor. This method was successfully applied to the highly sensitive measurement of a peptide hormone, B-type natriuretic peptide (BNP), with the use of the enzyme-labeled anti-BNP antibody. (C) 2004 Elsevier B.V. All rights reserved.