An important topic in the neurobiology of the affective disorders relates to changes in the metabolism of neurotransmitters. Since the early 1960s, the ‘serotonergic’ hypothesis (Coppen et al, 1972) has been supported by findings such as a low concentration of 5-hydroxy-indoleacetic acid (5-HIAA) in the cerebro-spinal fluid (CSF) of patients with melancholic depression and/or suicidal behaviour (Banki & Arato, 1983; Åsberg et al, 1984; Banki et al, 1984; López-Ibor, Jr et al, 1985; Korpi et al, 1986) as well as the efficacy of serotonin precursors, when combined with MAOIs, in the treatment of depressed patients (Coppen et al, 1963; Glassman & Platman, 1969; Ayuso et al, 1971; López-Ibor, Jr et al, 1976).