Symptoms of anxiety induced by 7.5% CO(2) inhalation can be attenuated by acute administration of GABA(A) receptor anxiolytics such as lorazepam and alprazolam. This study investigated if these effects are dose-related, by comparing a 0.5 mg dose (considered non-clinically effective) and a 2 mg dose of lorazepam (clinically effective) on 7.5% CO(2) inhalation. Eighteen healthy males (mean age 20.6 years, SD 1.29), judged physically and mentally fit, attended three visits, each one week apart, to take each treatment in a randomised double-blind crossover design. Drugs were given 60 min prior to 20 min air inhalation, followed by 20 min 7.5% CO(2) inhalation. The order of gas presentation was single blind. Subjective ratings using visual analogue scales (VAS) and questionnaires were recorded before and after each inhalation. Blood pressure (BP), heart rate (HR), respiration rate (RR) and expired CO(2) were recorded during each inhalation. Inhalation of 7.5% CO(2) significantly raised BP, HR, RR and expired CO(2) . Ratings of feeling like leaving the room were significantly lower on 2 mg compared with 0.5 mg and placebo, and dose-dependent trends were seen in scores for VAS fearful, anxious, stressed, tense, and worried. Results may be indicative of dose-dependent effects of lorazepam in a CO(2) model of anxiety. Copyright © 2012 John Wiley & Sons, Ltd.