Subjects will be selected from a group of adult men (18-65 years)based on the demonstration of a low testosterone level (<100 ng/dL) in association with low or inappropriately normal gonadotropin levels. All patients will undergo an initial assessment that includes an overnight 12-hour frequent blood sampling study to determine their degree of endogenous GnRH secretion. Following the overnight evaluation, subjects will have daily outpatient visits for 7 consecutive days when they will receive a GnRH bolus followed by 2hrs of blood sampling.