This project will firstly compare individuals with panic disorder who demonstrate abnormal brain transmitter (noradrenaline, serotonin) with those who demonstrate normal activity of these neurotransmitters, on interoceptive accuracy for changes in sympathetic nervous system arousal. It is predicted that individuals who demonstrate abnormal activity of these neurotransmitters will also demonstrate enhanced interoceptive ability to detect changes in arousal levels. Secondly, this project will evaluate the effects of a psychobiological treatment approach to panic disorder involving a combination of CBT and SSRI. This approach will be compared to the effects of both these interventions alone on a comprehensive array of measures related to panic disorder. It is predicted that the combined approach will be the most effective intervention on behavioural, psychological and biological variables, and cardiac risk. We will also be able to assess whether changes in panic disorder severity are associated with changes in interoceptive accuracy.