Human immunodeficiency virus-1 (HIV-1) gene expression is controlled by cellular transcription factors and by virally encoded trans-activation proteins of the HIV-1 tat and art/trs genes, which are essential for viral replication^1,9–11. Tat trans-activates HIV-1 gene expression by interacting with the trans-acting response element (TAR) located within the HIV-1 long terminal repeat (LTR) (ref. 2). In transient expression assays, tat mediates its effects largely by increasing the steady-state levels of messenger RNA species that contain the TAR sequence at or near their 5′ ends^2–4, suggesting a function for tat either in transcription or in subsequent RNA processing. The tat gene could also facilitate translation of mRNA containing the TAR sequence^5–8. To determine the mechanism of trans-activation by tat, we analysed the structure and rate of synthesis of RNA species directed by the HIV-1 LTR in transient expression assays both in the presence and absence of tat. Although the rate of HIV-1 transcription initiation was not affected by tat, transcriptional elongation beyond position +59 was seen only in the presence of tat. Thus, tat trans-activates HIV-1 transcription by relieving a specific block to transcriptional elongation within the TAR sequence.