The diagnosis, treatment, and pathophysiology of obsessive-compulsive disorder (OCD) were examined in a series of studies utilizing psychobiological approaches. Putative biological markers previously reported in depression were studied in this disorder and revealed that on some measures [Dexamethasone Suppression Test and rapid eye movement (REM) latency on sleep electroencephalogram (EEG)], OCD patients resemble those with major depressive disorder (MDD), whereas on others [REM density, platelet serotonin uptake, probably platelet 3H-imipramine binding, and 5-hydroxy-indoleacetic acid (5-HIAA) in cerebral spinal fluid (CSF)] they do not. The relationship between OCD and MDD was further explored in a double-blind, randomized crossover study designed to compare the antiobsessional effects of two tricyclic antidepressants, clomipramine (CMI) and desipramine (DMI), in a nondepressed cohort of OCD patients. CMI was found to have significant antiobsessional effects in this group, whereas in the same patients, DMI lacked therapeutic effects. These results suggest that not all antidepressants are antiobsessive and that some property of CMI, such as its potent serotonergic effects, may be of pathophysiological relevance for OCD. The role of serotonin in this disorder was then tested using the pharmacological challenge strategy. A novel serotonin postsynaptic receptor (5HT-1) agonist, m-chlorophenylpiperazine (m-CPP), was administered orally (0.5 mg/kg) under double-blind, placebo-controlled conditions to OCD patients and controls. In addition, a serotonergic receptor antagonist, metergoline (4 mg), was given to a subset of OCD patients. Relative to healthy volunteers, the OCD patients became significantly more anxious, depressed, and dysphoric after m-CPP administration. Moreover, in the OCD patients, obsessive-compulsive symptoms increased markedly after m-CPP and decreased significantly following metergoline administration. These results demonstrate that agents that bind to the 5HT-1 receptor can acutely affect the symptoms of OCD patients. The striking behavioral effects of these direct postsynaptic receptor ligands and the relative specificity of clomipramine as an antiobsessional agent suggest that serotonergic neurons may play a role in the pathophysiology, as well as mediating the pharmacological reduction, of obsessional symptoms.