5-Hydroxytryptamine (serotonin) was geometry optimized at the B3YP/6-31G(d) level of theory to determine the energetically most favourable conformations of the aromatic hydroxyl group and the protonated ethylamine side chain. The hydroxyl group was found to be most stable at anti for all conformations, and the two lowest energy gas phase conformers found were: chi(2) = g(+), chi(3) = g(-) and chi(2) = g(-), chi(3) = g(+). The protonated amino group was found equally stable at g+, g- and anti. The transition structures linking each gas phase minimum were also computed. Minima found were subjected to solvation calculations in chloroform, DMSO, ethanol and water, which shifted their relative stabilities. (C) 2002 Elsevier Science B.V. All rights reserved.