Structural studies on recombinant human monoamine oxidase A (hMAO-A) provides interesting insights on comparison with that determined for human MAO-B (hMAO-B) as well as comparison with that previously published for rat MAO-A. The active site cavity of hMAO-A is monopartite (as with rat MAO-A) while hMAO-B is a bipartite cavity. hMAO-A crystallizes as a monomeric form, in contrast to the dimeric forms exhibited by hMAO-B and rat MAO-A. All of the known MAO structures show nearly identical geometries around the covalent FAD sites. Differences in active site cavity structures occur away from the FAD site through conformational alterations (MAO-A's) and by changes in amino acid residues (hMAO-A and hMAO-B). Differences observed between human and rat MAO-A's raise questions regarding the appropriateness of the rat model in the development of MAO-A specific inhibitors as drugs for eventual human use.