This paper discusses the role of serotonin & its metabolite, 5-hydroxindoleacetic acid (5-HIAA), as markers of suicide proneness. Low levels of 5-HIAA have often been observed in endogenous depression & suicide attempts, especially the most serious ones. It stresses the similarity of platelets with serotonergic neurons, with the aim of considering the use of markers such as imipramine receptor sites in platelets. The latter would be much easier to measure than 5-HIAA in cerebrospinal fluid. (GW)