Intense exercise to exhaustion leads to increased susceptibility and severity of infections. T cells play an essential role in control of viral infections. While immune suppression is considered as a likely mechanism for exhaustive exercise-induced susceptibility to infection, we know little about viral-specific T cell response following exhaustive exercise in young or old mice. In this study, one group of female young (10-12 weeks) and old (22-24 months) C57BL/6 mice were exposed to a single bout of intense exercise to exhaustion and immediately infected with lymphocytic choriomeningitis virus (LCMV). Eight days later, at the peak of expansion phase of T cell response, we used tetramers of MHC class I molecules containing viral peptides to directly visualize antigen specific CD8 T cells and a sensitive functional assay measuring interferon- production at the single cell level to quantitate the CD8 and CD4 T cell response. To evaluate the impact of intense exercise during both the initiation and evolution of the expansion phase of the T cell response; a second group of young and old mice continued their daily bouts of intense exercise to exhaustion over the next 8 days. Our data shows that in young mice, LCMV infection following exhaustive exercise leads to suppression of LCMV specific CD8 and CD4 T cell responses and this suppression effect occurs at the initiation of the expansion phase of viral-specific T cells. However, in old mice, unlike young mice, exhaustive exercise does not cause suppression of LCMV specific T cell responses.