The purpose of this study is to determine whether there is a reliable relationship between the level of mirtazapine in the bloodstream and the occupancy of neuroreceptors by mirtazapine in the brain. We will apply our standard procedures of PET brain scanning and region-of-interest data analysis, using healthy volunteers who will receive a daily dose of mirtazapine (double-blind design with placebo, 7.5 mg or 15 mg daily for 5 days). We believe that this project could provide a procedure for assessing brain function in treatment-resistant depression, with the aim of improving the guidelines for successful, evidence-based treatment of depression.