The primary objectives of this study are to compare the virologic potency and pharmacology of TDF and ABC alone and in combination. Since it is not feasible or ethical to give mono or dual-therapy with these agents for prolonged intervals, this project was designed to take advantage of a short term drug exposure. The study performs intensive lab monitoring with a cross-over design to compare short courses of monotherapy and dual-therapy. This is an open-labeled study of a dual NRTI/NtRTI combination, ABC + TDF, compared to ABC and TDF monotherapy administered for 7 days. A total of 20 ARV-naïve subjects will be enrolled in this study. A screening genotype will be done to confirm that there are no resistance-associated mutations at baseline. Each subject will then be randomized to a 7-day sequence of monotherapy (ABC or TDF), and four measurements for plasma HIV RNA will be done to calculate the slope of the phase one viral decay. Prior to initiation of nucleoside analogues, PBMCs will be collected to measure baseline expression of nucleoside transport enzymes via RT-PCR and Western blot analysis. On days 7 and 8, serial blood specimens will be collected for plasma and intracellular levels of TDF and ABC. The monotherapy sequence will be followed by a 35-day washout period.