The location and nature of the HIV-1 packaging signal are largely unknown, despite several genetic and biochemical mutational analyses. In this report we present our attempts to define a minimal HIV-1 packaging signal through the generation of test RNAs containing small blocks of HIV-1 sequences. We constructed RNAs differing in the position and identity of the HIV-1 sequence and the segments of heterologous sequences. However, none of the vectors were efficiently, encapsidated by wild-type HIV-1 virions. These results contrast those of Moloney murine leukemia virus and Rous sarcoma virus, where small viral segments mediate the efficient encapsidation of heterologous RNAs. The results suggest that the HIV-1 packaging signal may be extremely dispersed or heavily context-dependent.