Human retroviruses, human T cell leukemia viruses (HTLV) and human immunodeficiency viruses (HIV), express two classes of mRNAs; fully spliced mRNA in the early phase and intron-containing mRNA in a later phase. The expressions of HTLV-1 rex and HIV rev by early mRNAs are essential for the later phase of expression of intron-containing gag and env mRNAs. Each two cis-acting sequences seem to be involved in these regulations: HTLV-1 rex depends on a splice donor (SD) and a responsible element (RXE) at the 3' end, whereas HIV rev depends on a specific repressive sequence (CRS) and a responsible element (RRE) in the intron, but does not require an SD. For analyses of these cis-acting sequences, we inserted an HIV element RRE into an HTLV-1 construct and tested the responses to HTLV-1 rex and HIV rev regulations. The results indicated that both rex and rev could regulate RNA expression of these chimeric constructs responding to an HIV RRE. A repressive element (CRS) was dispensable, and the intronic or exonic location of RRE was not important. These observations suggest that rex and rev could be functionally equivalent to induce cytoplasmic expression of unspliced RNA which expression is suppressed either by an SD or CRS depending on the construction.