The approved dose for AZT is 300 mg bid, and AZT is available as a 300 mg tablet or 100 mg capsule. A previous study in 10 HIV seropositive individuals comparing cellular AZT-mono, di- and tri-phosphate nucleotides at normal and reduced doses, 100 mg tid versus 300 mg bid, demonstrated a significant decrease in plasma AZT and intracellular AZT-monophosphate (AZT-MP) levels, (AZT-MP is associated with toxicity), without significant changes in AZT-triphosphate (AZT-TP) levels in activated PBM cells or antiviral activity. These findings on the effect of AZT dose on intracellular AZT-TP are supported by computer simulations. Maximal predicted cellular levels of AZT-MP and AZT-TP at steady-state, following 200 mg bid and 300 mg bid dosing in 3,000 simulated individuals demonstrated a high overlap between the AZT-TP histograms (> 85 %), suggesting similar efficacies for the 200 and 300 mg bid doses. The low degree of overlap between the AZT-MP histograms (< 8 %) for the two dose regimens, suggests that there may be fewer toxicities with the 200 mg bid dose, supporting our hypothesis that a zidovudine dose of 200 mg bid could reduce the AZT-MP levels without compromising the AZT-TP levels that would be obtained with a 300 mg bid dose.