Further Evidence: High Intensity Exercise Elevates Plasma Testosterone and Growth Hormone

A recent study by Shark, et.al. (Shark. 2010) found that a 30 minute (or time to exhaustion) submaximal 55' climbing route significantly increased plasma testosterone (T) and growth hormone (GH) in ten male rock climbers at the age of 21-30 years:

T concentrations significantly (p < 0.05) increased from Pre (6.04±0.31 ng/mL) to IP (7.39 ± 0.40 ng/mL), and returned to baseline at P15 [=15 minutes post exercise] (6.23 ± 0.33 ng/mL). Cortisol levels did not significantly change during the protocol. GH significantly (p < 0.01) increased from Pre (0.63 ± 0.17 ng/mL) to IP (19.89 ± 4.53 ng/mL), and remained elevated at P15 (15.03 ± 3.89 ng/mL).

In short: If you want to achieve an optimal anabolic endocrine response to exercise, keep the exercise duration short and the intensity high.