Caprine Arthirtis Encephalitis Virus (CAEV) and Visna Virus are two viruses of the lentivirus family. They encode three structural genes (gag, pol, and env) and two regulatory genes (rev and tat). The Rev protein regulates Gag, Pol and Env expression by transporting their mRNAs to the cytoplasm by binding to the RRE (Rev Response Element) found on their mRNAs. Previous studies have indicated that Rev may be toxic to transfected cells, overexpression of exogenous RREs or a better binding RRE can inhibit Rev activity and Rev-C (CAEV Rev) can trans-activate RRE-V (Visna Virus RRE). To test these possibilities FACS analysis, RNA binding assays, cotransfections, and SELEX were done. The results indicated that Rev is not acutely toxic to cells, inhibition of Rev activity could not be achieved by making a better binder or through expression of exogenous RREs, and Rev-C can trans-activate RRE-V implicating conservation of Rev/RRE interactions in lentiviruses.