The study will describe the short-term effects the study drug, rivaroxaban, has on the body when a patient is switched from enoxaparin injection (by needle) to oral rivaroxaban (by mouth) for the prevention of blood clotting in the veins after elective total hip or total knee replacement surgery. After providing written informed consent, screening procedures will be completed to assess eligibility. After enrollment, all patients will be switched from enoxaparin to rivaroxaban. Blood samples for the short-term effects of rivaroxaban will be taken at various times while in the subacute unit. At the time of discharge, if the study doctor feels it is appropriate, an adequate supply of rivaroxaban will be provided to complete the full course of therapy. Upon completion of rivaroxaban therapy, all patients will be required to have final study procedures performed. Safety evaluations at the final visit will include clinical blood laboratory tests, a physical examination, urine pregnancy test (if applicable), recording of any adverse events including details regarding any bleeding episodes or blood clot events, and assessment of the surgical wound. All patients will return any unused study medication and study participation will be complete.