- Mental disorders are highly prevalent and often difficult to diagnose. Although significant advances have been achieved in medication for mental disorders, the diagnosis and treatment monitoring of these disorders remain in a static situation. The absence of objective diagnostic 'gold standards', derives from the special complexity of diagnosis in psychiatry. Heterotrimeric G-proteins are crucial elements in post-receptor information transduction. These proteins have been implicated in the biochemical mechanism of action of drugs used to treat psychiatric disorders. G-protein measurements have unravelled a differential pattern characteristic of each of the major mental disorders. The accumulated data supports the potential use of G-protein measures as state-dependent markers for the biochemical diagnosis of mental disorders and as aid in the biochemical monitoring of the response to a specific treatment.