Study to Evaluate the Effects of Zemplar Injection and Calcijex on Intestinal Absorption of Calcium

Inclusion Criteria * Subject is ≥ 20 years of age. * Subject is diagnosed with ESRD, and must be on maintenance hemodialysis (HD) three times a week for at least 2 months prior to the Screening Phase and expected to remain on HD for the duration of the study. * If female, subject is either not of childbearing potential, defined as postmenopausal for at least 1 year or surgically sterile (bilateral tubal ligation, bilateral oophorectomy or hysterectomy), or is of childbearing potential and practicing one of the following methods of birth control: * Condoms, sponge, foams, jellies, diaphragm or intrauterine device (IUD) * Contraceptives (oral or parenteral) for three months prior to study drug administration * In a monogamous relationship with a vasectomized partner * If female, subject is not breastfeeding and has a negative serum pregnancy test prior to the treatment phase. * Subject had an intact PTH value \> 200 pg/mL. * Serum calcium level \< 10.2 mg/dL at Screening visit. * Serum phosphorus level \< 6.5 mg/dL at Screening visit. * Ca´P product ≤ 65 at Screening visit. * Must voluntarily sign and date an informed consent, approved by an Institutional Review Board (IRB)/Independent Ethics Committee (IEC), prior to the conduct of any study-specific procedure Exclusion Criteria * Subject has a history of an allergic reaction or significant sensitivity to vitamin D or vitamin D related compounds. * Subject has chronic gastrointestinal disease, which in the Investigator's opinion, may result in clinically significant GI malabsorption. * Liver function defects defined as \> 2 times the upper limit of normal for liver enzyme or \> 1.5 times the upper limit of normal coagulation levels. * Subject is taking maintenance calcitonin, glucocorticoids in an equivalent dose \> 5 mg prednisone, or other drugs that may affect calcium or bone metabolism, other than females on stable (same dose and product for 3 months) estrogen and/or progestin therapy. * For any reason, subject is considered by the Investigator to be an unsuitable candidate to receive pharmacological doses of vitamin D. * Subject has received any investigational drug within 4 weeks prior to the Treatment Phase.