In this article, I stated that St. John's wort is thought to work like an SSRI. The current feeling among researchers is that it exhibits properties of both a mild MAOI and an SSRI. The jury is still out, however, as to just what it's mechanism of action really is. Until such time that this becomes known, it is still wise to be aware of potential drug interactions between St. John's wort and other antidepressants. The prudent way to use it is to allow time for your current medication to be fully eliminated from your body (check with your doctor, pharmacist, or a good drug reference to determine this) prior to beginning St. John's wort.
Source:
http://depression.about.com/cs/altmedsjw/a/hypericum.htm