Here, we study the temporal expression of the inhibitory receptor programmed death 1 (PD-1) on simian immunodeficiency virus (SIV) Gag-specific T cells following pathogenic SIV infection or following vaccination with a DNA/modified vaccinia virus Ankara (DNA/MVA) vaccine and simian/human immunodeficiency virus (SHIV) challenge in macaques. Following infection, the majority (>95%) of Gag-specific CD8 T cells expressed PD-1, and the level of PD-1 expression per cell increased over time. The level of PD-1 expression in lymph nodes and rectal mucosal tissue, the major sites of virus replication, was higher compared to blood. In vitro blockade of PD-1 resulted in enhanced proliferation of SIV-specific CD8 as well as CD4 T cells. In contrast, following vaccination, the majority of peak effector Gag-specific CD8 T cells expressed low levels of PD-1, and these levels decreased further as the cells differentiated into memory cells. In addition, following SHIV challenge of these vaccinated macaques, the level of PD-1 expression on Gag-specific CD8 T cells correlated positively with plasma viremia. These results demonstrate that SIV-specific CD8 T cells express PD-1 after exposure to antigen but downregulate expression under conditions of antigen clearance and enhance expression under conditions of antigen persistence. They also demonstrate that the level of PD-1 expression per cell rather than the presence or absence of expression plays an important role in regulating CD8 T-cell dysfunction in pathogenic SIV infection. In addition, they demonstrate that similar to HIV infection, the PD-1:PD-1 ligand inhibitory pathway is operational in pathogenic SIV infection, and the macaque/SIV model would be ideal to test the safety and therapeutic benefit of blocking this pathway in vivo.