Maraviroc is the only CCR5 antagonist currently approved by the United States Food and Drug Administration, the European Commission, Health Canada. Maraviroc (Selzentry, Pfizer) is a small molecule currently approved for treatment of patients infected with R5-tropic HIV-1. It is metabolized by CYP3A4. The dose may have to be adjusted when given with CYP3A4 inducers or inhibitors, primarily drugs that are also used for HIV therapy, but also for several anticonvulsants. None of the subjects in the proposed trial will be entered if on these medications. The drug has a good pharmacokinetic profile with relatively low protein binding and high bioavailability when given at standard doses twice a day. It is moderately lipophilic, so it can penetrate the blood-brain barrier. At a single dose of 300 mg, time to maximum concentration occurred by two hours post-treatment in humans. The terminal half-life is 14-18 hours, so a single dose used during the time of training in the proposed protocol should be adequate, rather than the BID treatment for AIDS. Despite low CSF concentrations, the drug suppresses CSF viral load. It potently inhibits downstream CCR5 signaling and does not induce CCR5 internalization, suggesting that the drug is a functional CCR5 antagonist.