Inclusion Criteria:Inclusion criteria: CKD subjects; males with calculated GFR (MRDR equation) between 15 and 40 ml/min/1.73m2 and stable or slowly progressive renal failure (decline in function of <1ml/min/month) including those patients requiring hemodialysis and serum testosterone levels of <300 ng/ml and capable of safely performing required exercise testing and serum testosterone levels of <300ng/ml and capable of safely performing required exercise testing. Control subjects; good health, normal serum creatinine levels, normal TT levels and able to perform required exercise testing safely. The racial and ethnic composition of the subjects will reflect the composition present in the ESRD population in the counties in Northern California from which our patients are referred. Subjects to be of age 45-80 years.

Exclusion Criteria:Exclusion criteria: applicable to both CKD and control subjects. Any unstable chronic medical condition, previous kidney transplant. Uncontrolled diabetes mellitus, active vasculitis, active autoimmune disease, malignancy(<5 yrs), obesity (BMI > 35), alcoholism or other recreational drug use, active heart disease, angina, uncontrolled arrhythmias or myocardial infarct within past 3 months, peripheral vascular disease with claudication, active lung, liver or GI disease, sleep apnea, medically unstable subjects and subjects who received anabolic, catabolic or cytotoxic medications during the prior 3 months. History of prostate CA, PSA >4g/ml, or advanced BPH (AUA symptom score > 21) and abnormal prostate on digital rectal examination. Bone or joint abnormalities that would preclude exercise testing.