Human immunodeficiency virus type 1 resistance to nucleoside reverse transcriptase inhibitors such as 3'-azido-2', 3'-dideoxythymidine (AZT) can arise through mutations in the coding region of reverse transcriptase (RT) that enhance the enzyme's ability to remove the drug after it has been incorporated. This excision activity of HIV-1 RT has been well characterized in a number of in vitro systems. However, the in vitro findings do not provide a complete picture of the in vivo significance of this resistance mechanism. This review will attempt to bridge the gap between the in vitro observations and the in vivo environment by summarizing the fragmentary information that is available about the intracellular conditions that may influence drug excision in cell subpopulations that are infected by HIV-1. Topics that will be discussed include (a) intracellular compounds HIV-1 RT may use to remove chain terminators; (b) how dNTPs can affect excision activity and how these effects differ in different immune cell subpopulations; (c) the influence of HIV infection on excision activity--e.g., through immune activation of infected cells or through changes indirectly induced in cells that subsequently become infected; (d) intracellular conditions that favor selection for mutations that increase the excision-based resistance mechanism; (e) the importance of macrophages in the selection of resistance mutations. Understanding factors that control excision in the intracellular environment will greatly enhance our understanding of the process of selection for this class of drug resistance mutations and may open doors for the development of novel targets for antiviral therapy.