We have compared the effects of nucleoside analogs in quiescent and phytohemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC) exposed to human immunodeficiency virus type 1 (HIV-1) with those of their triphosphorylated derivatives in cell-free HIV-1 reverse transcription assays. We observed a substantial decrease in synthesis of early minus-strand proviral DNA products in HIV-1-infected, quiescent PBMC exposed to each of 3'-azido-3'-deoxythymidine (AZT), 2',3'-dideoxyinosine (ddI), and 2',3'-dideoxy-3'-thiacytidine (3TC), in comparison with nontreated, infected controls. In contrast, no such diminution was observed when PHA-stimulated, HIV-1-infected PBMC were treated with the same drugs. This result was attributed to previously reported findings that PHA-stimulated PBMC possessed larger deoxynucleoside triphosphate (dNTP) pools than quiescent cells did. To further investigate this subject, a cell-free HIV-1 reverse transcription reaction involving HIV-1 RNA genomic template, recombinant purified HIV-1 reverse transcriptase, all four dNTPs and either tRNA3Lys or a deoxyoligonucleotide as primer was used to monitor chain termi-nation mediated by 2',3'-dideoxynucleoside triphosphates (ddNTPs) during synthesis of minus-strand strong-stop DNA. Augmented chain termination was observed with decreasing concentrations of both ddNTP and dNTP when the ratio of dNTP to ddNTP was fixed. We also found that both the number and strength of reverse transcription pause sites were increased at low concentrations of dNTPs and when a deoxyoligonucleotide primer was used in place of the cognate primer, tRNA3Lys. Preferential incorporation of ddATP was observed dur-ing reverse transcription opposite a distinct pause site in a short synthetic RNA template. These results con-firm the notion that the antiviral activities of ddNTP are dependent on both cellular dNTP pools and the state of cellular activation. Pausing of HIV-1 reverse transcriptase during reverse transcription, altered by dNTP concentrations, may be a mechanism that controls the position and extent of incorporation of nucleoside analogs.