Refined quantum-mechanical computations invariably predict that the preferred conformers of free androgenic phenethylamines or hallucinogenic indolalkylamines should correspond to values of the torsion angles rt---+ 90 ° and rz close to + 60 or 180 °. X-ray crystallographic studies indicate that most such compounds exist in these conformations (in particular with rl ~ 90, r2-~ 180 °) in the crystals. In some cases, however, the crystalline conformer corresponds to rl -~ 0, r2-~ 180 °, an arrangement which does not even correspond to a local energy minimum on the conformational energy map for the free molecule. Such is, for example, the case for adrenaline in adrenaline hydrogen tartrate. Computations carried out for the lattice energy of this crystal and of the hypothetical crystals constructed with the usual conformers, by a procedure which uses intermolecular potential functions, show that the lattice energy of the 'experimental' crystal largely compensates for the loss in conformational energy of the constituent unit and represents a more stable arrangement than those obtained with conformers associated with rl-+ 90 °.