Transcriptional activation of gene expression directed by the long terminal repeat (LTR) of the human immunodeficiency virus type 1 (HIV-1) requires both the Tat activation response element (TAR) and the Tat protein. Mutants lacking a functional tat gene are not able to replicate. An approach we have used to suppress HIV-1 gene expression is based on the controlled overexpression of multimerized TAR sequences, which results in the sequestration of one or more components of the Tat response. Since Tat has no known cellular analog, a modified HIV-1 LTR, which is highly induced by the presence of Tat, was used to promote the expression of the multimerized TAR (poly-TAR) specifically in the presence of Tat. Cotransfection of an HIV-1 LTR-controlled poly-TAR plasmid with LTR-Tat and LTR-CAT plasmids inhibited the level of the reporter gene activity (CAT) as much as 97%. The downregulation of HIV-1 gene expression observed was dependent on the quantity of transfected poly-TAR as well as the number of tandem TAR repeats expressed per unit transcript. Similar constructs lacking either LTR upstream sequences or the TAR sequence had no significant effect, suggesting that the competitive effect was mediated at the RNA level and that it was the nascent RNA, rather than DNA, that was recognized by the Tat protein. Tat-regulated production of the poly-TAR transcript provides a means for dissecting the mechanism of Tat-mediated trans-activation of the HIV-1 LTR. The ability to regulate a viral inhibitory gene so that it is expressed only when needed should prove useful in devising an antiviral strategy through gene therapy.