The study will determine the effects of D-Cycloserine (DCS) on fear reduction in patients diagnosed with phobia of heights (acrophobia) undergoing one session of three hours of virtual reality exposure therapy (VRET) or in vivo exposure therapy (IVET).

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, N1=20); one-session VRET (3 hours) and placebo (VR-OST + Pl, N2=20); one-session IVET (3 hours) and placebo (IV + Pl, N3=20); or one-session IVET (3 hours) and 50 mg D-Cycloserine, (IV + DCS, N4=20). 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.

Inclusion Criteria:

• Age between 18 and 65 years
• Men and women
• Diagnosis of acrophobia based on a clinical interview (ADIS-IV)
• Must sign an informed consent document after complete explanation of the study documenting that they understand the purpose of the study, procedures to be undertaken, possible benefits, potential risks, and are willing to participate.

Exclusion Criteria:

• A lifetime history of bipolar disorder, schizophrenia, psychosis, delusional disorders; organic brain syndrome, cognitive dysfunction that could interfere with capacity to engage in therapy;
• A history of substance or alcohol dependence (other than nicotine) in the last 6 months or otherwise unable to commit to refraining from alcohol use during the period of study participation.
• Patients with significant suicidal ideation or who have enacted suicidal behaviors within 6 months prior to intake will be excluded from study participation and referred for appropriate services.
• Patients must be off concurrent anxiolytics and beta blockers and on a stable dose of antidepressant medication for at least 3 weeks prior to initiation of randomized treatment.
• Patients receiving medication that might interfere with study medication (medication that might lower seizure threshold: meperidine or antibiotics in high dosage: penicillins, cephalosporins, amphotericin and imipenem),
• Patients with a current or past history of seizures
• Pregnant women, lactating women, and women of childbearing potential who are not using medically accepted effective methods of contraception or abstinence
• Patients with a history of renal insufficiency (creatinine clearance less than 60 mL/min) or liver insufficiency
• Any concurrent psychotherapy initiated within 3 months of baseline, or ongoing psychotherapy of any duration specifically targeting phobias
• Patients unable to understand study procedures and participate in the informed consent process.
• Other serious medical illnesses (e.g., cardiovascular, liver, kidney, respiratory, endocrine, neurologic, or blood-related diseases),
• Inability to tolerate wearing the Virtual Reality Head Mounted Display,
• If patients refuse the study medication
• Any allergic reactions to D-Cycloserine by history