The Rev proteins of primate immunodeficiency viruses are essential transactivators to switch from early to late phase in the viral replication cycle. Surprisingly, the Rev protein of HIV-1 is able to substitute those of HIV-2 and, as shown in here, of SIVmac239, but not vice versa. To understand the underlying mechanism of this incomplete functional reciprocity, we constructed a series of chimeric HIV-1/SIVmac239 Rev proteins and tested for transcomplementation efficacy on Rev-dependent indicator plasmids. In addition, we analyzed the prokaryotically expressed wild type and chimeric proteins for RNA-binding properties in a gel-shift assay in vitro. The functional defect of SIVmac239 on the HIV-1 Rev response element (RRE) is not due to a lack of binding or multimerization. In cotransfection experiments, SIVmac239 Rev and the chimeric proteins were tested for potential inhibitory effects on HIV-1 Rev function using the HIV-1 based indicator plasmid. Some of these proteins turned out to be transdominant inhibitors almost as potent as the HIV-1 Rev mutant M10 which is localized in the activation domain and is one of the strongest transdominant inhibitors. Surprisingly, M10 was not able to inhibit the function of either Rev protein on SIVmac239 RRE, whereas a corresponding SIVmac239 Rev mutant (SIV M10) was a transdominant inhibitor of SIVmac239 Rev function on its homologous RRE.