This study would be a prospective cohort study, stratified by weight. Patients who are stable on nevirapine for at least 3 months prior to study entry, but no more than 1year will be recruited. For 1 week, subjects will take their morning dose of nevirapine in the clinic (and be observed taking it), and given their evening dose to take at home. Subjects will need to record the date and time they take their evening dose. At the end of the week, the subjects will be sequestered at the clinic for blood draws for a 12hr pharmacokinetic sampling. They will not be brought back in at 24hrs as they will need to be redosed after 12hrs. They will have Liver Function Tests (LFT's) and other labs drawn at study entry, and one month after the end of study. Pharmacokinetic levels will be analyzed using standard mathematical models, and Area Under the Curve (AUC), Maximum concentration (Cmax), and Minimum Concentration (Cmin) will be compared between groups.