The aim of the present study was to evaluate neuroendocrine and behavioural responses to a challenge with the selective serotonin (5-HT) reuptake inhibitor citalopram (Cit) in patients with social anxiety disorder (SAD). Cit was given intravenously (20 mg over 30 min) to 18 patients with SAD and 18 matched healthy subjects in a double-blind, placebo-controlled cross-over design. Cit challenge resulted in the increased plasma concentrations of cortisol and prolactin relatively to placebo without significant differences between the patients and controls. The patients had higher ratings of anxiety that were not affected by Cit, and more headaches than controls after Cit. Thus, the neuroendocrine sensitivity to 5-HT stimulation with Cit in patients with SAD was not different from the response in controls indicating lack of major alterations in the function of 5-HT receptors. The increased headache in patients may suggest hypersensitivity of some subtypes of 5-HT receptors in SAD.