To the Editor: We appreciate the comments by Imre Janszky, M.D., and Maria Kopp, M.D., Ph.D., and welcome the opportunity to clarify our data. Indeed, the role of anatomical (and functional) dead space may be of particular relevance in the respiratory physiology of panic disorder patients. For instance, as we have shown elsewhere (1) that the diminished ability of panic patients to expel CO2 following tryptophan depletion may be related to anomalies in dead space. It is true that, because of dead space, increasing respiratory rate with unchanged or decreased tidal volume "panting," as a general rule of physiology, will diminish the acid-base effects of hyperventilation in subjects without significant pulmonary pathology. While panting clearly works for dogs in hot weather, the hypothesis that low-tidal-volume hyperventilation using dead space is a successful coping mechanism for panic disorder patients in response to anxiogenic situations is not supported by our data. First, the difference in tidal volumes between panicking and nonpanicking patients during the hyperventilation period was not significant. Second, if the patients are divided into panickers (N=16) and nonpanickers (N=38) according to self-rating during the hyperventilation period (in table 2, p. 1560, panic rating is based on self-rating during 5% CO2 inhalation), the difference in tidal volumes is reversed (panickers: 359 ml; nonpanickers: 388 ml; n.s.). The suggestion that without the instruction to maintain a respiratory rate of 30 breaths per minute nonpanicking "panters" would increase their respiratory rate the most is again unlikely in view of our data. While it is possible that CO2- and hyperventilation-induced panics involve different mechanisms, we found that it was the panicking group that increased respiratory rate the most during CO2 challenges. It is not surprising that successful breathing retraining is based on instructing panic disorder patients to slow their respiratory rate and learn to adjust tidal volume to meet their metabolic needs.