The presence of cholinesterase at the myoneural junction of intercostal muscle has been demonstrated in both light and electron microscopic preparations. A new simultaneous diazo coupling technique using α-naphthyl acetate as substrate and "hexazonium pararosanilin" as coupler has been applied to cold formalin-fixed tissues. After postfixation in buffered osmium tetroxide the sites of esterase activity are faithfully demonstrated at a high level of resolution. The details of cholin-esterase distribution and some technical aspects of the procedure are discussed.

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