A method is described for the bioassay of acetylcholine (ACh) using a standard digital balance to which a strip of eserinized frog rectus abdominus muscle is attached directly in a 0.6-ml perfusing bath. Dose–tension curves were linear between about 10 and 100 ng of ACh with 10–15 mg increment in tension per nanogram of ACh in the bath. With 60 s allowed for contraction, samples can be tested each 4 min, with good reproducibility for 6–8 h.