Patients undergoing hip replacement often experience moderate to severe acute pain post-surgery. Normally such pain is controlled when patients receive repeated doses of opioid analgesics. However, opioid therapy is commonly associated with side effects such as nausea, vomiting, sedation, constipation, addiction, tolerance, and respiratory depression. CG5503, a newly synthesized drug also acts as a centrally acting pain reliever but has a dual mode of action. The aim of this study is to investigate the effectiveness (level of pain control) and safety (side effects) of 3 dose levels of CG5503, in an immediate release, (IR) formulation, compared with no drug (placebo) or one dose level of oxycodone (an opioid commonly used to treat post-surgical pain). This study is a randomized, double-blind (neither investigator nor patient will know which treatment was received), active- and placebo-controlled, parallel-group, multicenter study to evaluate the treatment of acute pain from hip replacement surgery. The study will include a blinded 72 hour in-patient phase immediately following hip replacement surgery, during which patients will be treated with either 50-, 75-, or 100-mg CG5503 base IR, a placebo, or 10-mg oxycodone, and pain relief will be periodically assessed. Following this phase, patients wishing to continue treatment with CG5503 IR may enter an outpatient voluntary nonrandomized, open-label extension phase for 9 days during which they will receive 50- or 100-mg CG5503 IR. Assessments of pain relief include the pain intensity numeric rating scale (PI), pain relief numeric rating scale (PAR) and patient global impression of change scale (PGIC). Safety evaluations include monitoring of adverse events, physical examinations, and clinical laboratory tests. Venous blood samples will be collected for the determination of serum concentrations of CG5503 and oxycodone. The null hypothesis for the study is that efficacy results for all CG5503 IR dosage groups are equal to placebo based on the mean sum of pain intensity difference at 48 hours. The alternative study hypothesis is that at least 1 dose strength of CG5503 will be different from placebo in controlling pain at 48 hours. CG5503 base IR 50, or 75, or 100 mg, or oxycodone 10 mg, or placebo, 1 capsule taken by mouth every 4 to 6 hours during the 72 hour postsurgery phase of the study (one extra dose is allowed, if needed for pain); and CG5503, 50 mg base capsules, 1 to 2 tablets taken by mouth every 4 to 6 hours for up to 9 days during the open label portion of the study. All doses of study treatment will be taken with approximately 120 mL of water with or with food.