Why does an anti-depressant work for some people, but not others? One of the mysteries of psychopharmacology has been accounting for the two- to four-week latency of antidepressant response after a depressed patient starts taking the medication. A complicated cascade of neurochemical changes has to occur, resulting in altered brain functioning. Now a team of Toronto neurologists using positron emission tomography (PET scanning) have demonstrated a progressive sequence of changes in the brain function of patients in those weeks and the absence of those predictable changes in patients who will turn out to be nonresponders to the antidepressant studied, fluoxetine (Prozac). PET scanning visualizes levels of brain activity in various regions by imaging rate of glucose metabolism. The hippocampus, a “hot” region in current neuropsychiatric research on a variety of disorders, was one of the areas implicated.