Recent clinical and laboratory studies have suggested that changes in brain serotonin (5-HT) function may contribute to anxiety symptoms and anxiety-type behaviors. Among the anxiety disorders, perhaps the most compelling evidence implicating 5-HT exists for obsessive compulsive disorder (OCD). In controlled trials, patients with OCD were markedly more responsive to treatment with 5-HT-selective uptake inhibitors such as clomipramine, fluvoxamine, or fluoxetine than to norepinephrine-selective or nonselective uptake inhibitors or to other psychoactive drugs. Studies with 5-HT agonists and antagonists also support a role for 5-HT in OCD. In this review, pharmacologic studies involving 5-HT-selective therapeutic and anxiogenic agents and non-5-HT-selective anxiogenic agents in patients with OCD are compared and contrasted with similar studies in patients with anxiety and panic disorder.