100 pregnant HIV-1 seropositive women in Nairobi with CD4 counts between 200 to 500 who have chosen to breastfeed will receive either ARV regimen. Mother-infant pairs will be followed for 1 year after delivery. Home visits will be conducted in the first month (~10 visits) to collect 2-5 mls of breastmilk per visit. Mother-infant pairs will be seen in the study clinic with maternal blood and breastmilk and infant blood collected at months 1, 3, and 6 for HIV-1 and HIV-1 Elispot assays. Breastmilk HIV-1 RNA and DNA levels will be quantified in Dr. Overbaugh's laboratory in Seattle and Elispot assays conducted in Nairobi with validation of a subset in Dr. Rowland-Jones laboratory in Oxford. Viral loads, decay curves, half-life, and re-population following ARV cessation will be estimated for each regimen and regimens compared. These studies will provide insight into the viral and immune responses to ARV regimens proposed for prevention of breastfeeding HIV-1 transmission and will be important for rational design of future interventions. After taking into account, estimated loss to follow-up, the targeted sample size with outcome data was 80 women, 40 in each trial arm, estimating undetectable breast milk HIV-1 RNA levels in the HAART arm and median breast milk HIV-1 RNA levels of 3.0 log10 in women receiving ZDV/NVP.