BACKGROUND: A key clinical issue is how to maximise the belief change central to cognitive therapy. Physiological arousal is a key internal cue confirming threat beliefs in anxiety disorders. Deeper extinction of anxiety may occur if catastrophizing responses to physiological arousal are inhibited prior to joint exposure with external phobic stimuli. The aim of the study was to test whether increasing physiological arousal using exercise increases the benefits of behavioural tests. METHODS: Sixty individuals with a fear of heights had one session of VR cognitive treatment. They were randomised to have the treatment either with periods of intense physical exercise (cycling at 80% of maximum heart rate) prior to exposures or without. Linear mixed effects models were used to check the manipulation and test the primary hypothesis of a group difference in degree of conviction in the phobic threat belief. RESULTS: Heart rate was significantly higher in the exercise group throughout compared with the control group. Both groups showed significant reductions in threat beliefs after the VR treatment (d = 1.0, p