Summary-Describedare the synthesis and some aspects of the pharmacology of acetylseco hemicholinium-3 (acetylseco HC-3), the acetylated open ring analogue of hemicholinium-3 (HC-3). The effects of both compounds were determined in uiuo on rat brain acetylcholine (ACh), 'Y-choline (14C-Ch) incorporation into 14C-acetylcholine (14C-ACh) and on one way jump box avoidance and escape behavior in naive and trained rats. In addition, the in vitro effects of both drugs were determined on choline acetyltransferase activity (ChAc) in rat brain.When given intraventricularly in doses of l-20 pg both compounds reduced total ACh content in the brain to a maximum of 5'0% of normal in 30-60 min. In doses of 20 pg intraventricularly, both drugs also reduced 14C-Ch incorporation into Y-ACh by 845% for acetylseco HC-3 and by 52% for HC-3.The in viva changes of ACh in the brain were correlated with the behavioral deficits induced in one way shuttle box acquisition and retention. In doses of 20 pg total intraventri~ularly, both compounds produced behavioral deficits which were greater in naive than in trained animals. Zn vitro, acetylseco HC-3 inhibited ChAc activity with an 150 of 1 x 10e5 M with Ch IO-* M and acetyl CoA 6.4 x 10m4 M, while HC-3 had no inhibitory effects. Using rat brain homogenate as the enzyme source and commercial acetyl CoA for kinetic studies, acetylseco HC-3 was shown to be a mixed inhibitor of acetyl CoA and a competitive inhibitor of Ch.The in uiuo actions of acetyiseco HC-3 are consistent with those of a ChAc inhibitor. However, it is necessary to rule out the possibility that the drug may also compete with Ch for its transport across biological membranes like its deacetylated derivative HC-3.The hemicholiniums have been widely investigated since they were first synthesized in 1954 by LONG and SCHUELER. Hemicholinium-3 (HC-3) is the prototype compound of this series. This agent reduces tissue acetyfcholine (ACh), possibly by reducing the active transport of choline (Ch) as shown by MACINTOSH (1963) and HODGKIN and MARTIN (1965). However, other mechanisms such as a shift in Ch metabolism toward phospholipid formation (G~MEz, DOMINO and SELLINGER, 1970a,b;GOMEZ, SELLINGER, SANTIAGO and DOMINO, 1971) and production of a false neurotransmitter through acetyhttion (RODRIGUEZ DE LORES ARNAIZ, LIEBER and DE ROBERTIS, 1970) have been proposed. SCHUELER (1955) found that WC-3 was the most toxic of some 20 bis-quaternary derivatives studied. HC-10 or acetylseco hemicholinium-3 (acetylseco HC-3) produced a toxicological picture similar to HC-3. SCHUELER assumed that acetylseco HC-3 was hydrolyzed in viuo to HC-3. There was little evidence then that acetylseco HC-3 was pharmacologically different from HC-3.