This study will measure serum levels of the active Vitamin D compound that circulates in hemodialysis subjects treated with either doxercalciferol injection (Hectorol®) or Zemplar® (paricalcitol injection).

Inclusion Criteria:

• The subject must be receiving hemodialysis three times per week for a minimum of three months.
• The subject must be receiving intravenous doxercalciferol or paricalcitol at each dialysis session (three times weekly) for a minimum of 12 weeks prior to Screening.
• At Screening Visit the subject's laboratory measurements must be within the following ranges: Serum iPTH measurement between 150-800 pg/mL; Corrected calcium measurement ≤ 10.5 mg/dL; Serum phosphorus measurement ≤ 7 mg/dL

Exclusion Criteria:

• Any ongoing use of over the counter or prescription vitamin D preparations except doxercalciferol or paricalcitol injection or multivitamins.
• History of heparin-induced thrombocytopenia.