The effect of a short treatment (7 days) with citalopram on the reactivity to inhalations of 35% CO2 and 65% oxygen and on clinical symptomatology was investigated in 15 patients with panic disorder who had a positive response to 35% CO2 inhalation. An open study design was applied. On day 0, before starting drug treatment, and after 1 week of treatment, each patient underwent the 35% CO2 challenge, and clinical symptomatology was evaluated with psychometric scales. The results showed a significant reduction of CO2 reactivity and of scores on the anticipatory anxiety subscale of Panic Associated Symptoms Scale. These results confirm that the serotonergic system plays an important role in the modulation of CO2 hyperreactivity and suggest an early anxiolytic effect of citalopram in patients who have panic disorder and are hyperreactive to CO2.