The subproject 1.5 "Neurobiology of Suicidal Behavior" is a multicenter study assessing peripheral parameters of the serotonergic, noradrenergic, and dopaminergic transmitter systems. Additionally, stress hormones and the lipid system as well as inhibitory and excitatory amino acids will be investigated. The different parameters are collected in cerebral spinal fluid (CSF), blood, and saliva. Patients with a depressive spectrum disorder with and without a suicide attempt (during the last three weeks) and being medication free for two weeks are included in the study. So far, 103 patients and controls have been recruited. The design and development of this project as well as interconnections with the others subprojects are described. Preliminary results about the stress hormone system and suicidality are presented.