 The impact of D-Cycloserine on Virtual Reality Exposure Therapy VRET and In Vivo Exposure Therapy IVET for phobia of heights acrophobia will be tested using a randomized, double-blind, placebo-controlled trial. Eighty participants will be randomly assigned to one of the four treatment conditions: one-session VRET 3 hours and 50 mg D-Cycloserine VR-OST DCS, N120; one-session VRET 3 hours and placebo VR-OST Pl, N220; one-session IVET 3 hours and placebo IV Pl, N320; or one-session IVET 3 hours and 50 mg D-Cycloserine, IV DCS, N420. For twenty participants 5 from each group the treatment will be delayed for three weeks and an additional assessment will be conducted as they will comprise a Wait List WL control group. Behavioral, cognitive and physiological changes in acrophobia will be assessed at pre-treatment, post-waitlist, post-treatment and 3 months follow-up. 