I knew I had a book around here somewhere that covered this. I found it and it says this. First of all, it is a widely held misconception that exercise increases testosterone levels. In fact, prolonged exercise decreases the production of testoterone by approximately 10%. Many studies have reported post-exercise increases in serum test levels, however, what they're mearuring is only temporary false elavations. This is because exercise causes significant decreases in plasma volume, and the metabolic clearance rate of testoterone.