To date, the majority of microbicide research has focused on the assessment of the safety and effectiveness of vaginal microbicides used for the prevention of HIV transmission via the vaginal compartment. Receptive anal intercourse (RAI) is common among men who have sex with men (MSM), and there is increasing evidence that heterosexual women in the developed and developing world also practice anal sex. It can, therefore, be anticipated that once vaginal microbicides are licensed, they will be used in both the vaginal and rectal compartments. As a consequence, there is a need to evaluate both the rectal and vaginal safety profile of candidate microbicides. Therefore, the primary objective of this study is to evaluate the systemic safety of 1% vaginally formulated tenofovir gel applied rectally. In addition, this study will evaluate the immunotoxicity of the gel and evaluate its acceptability; it will also use the oral tenofovir disoproxil fumarate tablets (TDF), rectally-applied tenofovir gel,and a placebo gel to compare their systemic and compartmental pharmacokinetic (pK) profiles.