It has been demonstrated that human immunodeficiency virus type 1 (HIV-1) virions are biochemically active particles, within which reverse transcription can take place even in physiological microenvironments. This process has been termed "natural endogenous reverse transcription" (NERT). In this report, we demonstrate that purified virions of simian immunodeficiency virus (SIV) also contain virus-specific DNA, which resulted from partial reverse transcription. Further, viral DNA synthesis could be initiated in SIV virions in the presence of polyamines and deoxyribonucleoside triphosphates (dNTPs), at physiological concentrations. The viral infectivity upon initially quiescent cells was significantly increased, when the levels of intravirion reverse transcripts were modulated. These data suggest that NERT of SIV may play an important role for SIV pathogenesis and transmission. In contrast to HIV-1, these hypotheses may be further directly investigated by in vivo model systems.