The action of various salts of acetylcholine on isolated mast cells from Wistar and F1 hybrids of Wistar and August rats was investigated. None of the acetylcholine salts within the concentration range 10(-12) M to 10(-2) M was able to release histamine either from Wistar or from hybrid mast cells. Compound 48/80, used as a control, was active in both cases. The results obtained are in opposition to some recent reports. The possible reasons for these contradictions are discussed.