Panic disorder and agoraphobia have been postulated to occur when (1) fear is elicited by some automatic mechanism that requires catastrophic cognition, (2) there is a flaw in the physiology of fear, with special reference to the noradrenergic system, or (3) a putative suffocation alarm mechanism sends out false alarms. The presence of a suffocation alarm system has been supported by studies of children who lack this protective mechanism because they suffer from congenital central hypoventilation syndrome. Antidepressants with serotonin activity seem to control panic disorder by down-regulating the suffocation alarm system. Serotonin selective reuptake inhibitors (SSRIs) are among the most effective drugs for panic disorder, emphasizing the role of serotonin in respiratory regulation. Dyspnea and hyperventilation are the cardinal signs of a panic attack. Because carbon monoxide (CO) does not cause panic, it may sabotage the suffocation alarm system by acting as an inhibitory neurotransmitter within the carotid body.