This study will evaluate platelet function in patients during treatment with selective serotonin reuptake inhibitors (SSRIs) which are widely used antidepressant agents. SSRIs, which are known to decrease platelet serotonin content, have been reported to be associated with bleeding in a minority of patients and recently have been associated with an increase in gastrointestinal bleeding. The purpose of this study is to better understand the potential risks of bleeding associated with mild platelet dysfunction in patients using SSRIs and to determine whether a global test of platelet function, as performed on the platelet function analyzer-100, is able to identify the changes in platelet function associated with SSRI use. Until recently, platelet function testing has utilized platelet aggregation studies which are very labor-intensive and require highly skilled technicians; a newer instrument, the Platelet Function Analyzer-100 (PFA-100, Dade Behring, Deerfield, IL) requires much less technical input to evaluate platelet function. The patients will be selected from a private clinic in the Washington, DC, area and will undergo a baseline assessment (history of bleeding and/or thrombosis), blood tests for routine blood counts, chemistries, and platelet function using the PFA-100. The analysis will include a statistical comparison of the platelet function in patients on SSRIs with those on a non-SSRI antidepressant medication (bupropion). It will also include an analysis of the results of the PFA-100 compared to the results of platelet aggregation. The results of the study may provide a better understanding of platelet function and screening for platelet function defects in patients using SSRIs.