SummaryA microbioassay for acetylcholine is described in which a solution containing acetylcholine is microperfused onto the endplate area of a chronically denervated skeletal muscle fiber. The endpoint, a brief tetanus, can be produced by application of as little as 0.15 μl of a solution which contains acetylcholine chloride at a concentration of 2.5 × 10-8 moles/1.