AbstractAb initioSCF‐LCAO‐MO computations using a minimal basis set have been carried out on the acetylcholine ion in different conformations. The rotational barrier around the C9‐C8‐O1‐C2 dihedral angle (τ1) has been also calculated: the curve presents a broad minimum. SCF calculations have been carried out on the system Ach‐water at 468 different separations and/or orientations; the interaction energies were then fitted to a pair potential function used in Monte Carlo (MC) simulations of Ach water solution. The rdfs indicate a global first hydration shell with about 38 waters. Details about the hydration shell are presented. The overall MC results indicate that for the Ach ion in water solution at room temperature relative free rotation is possible around τ1, and that Ach is well represented as a large positively charged hydrated globe with a small mobile tail free of strong interactions with the surrounding wa