In vitro studies have shown that naive CD8+ T cells are unable to express most of their effector proteins until after at least one round of cell division has taken place. We have reassessed this issue in vivo and find that naive CD8+ T cells mount Ag-specific responses within hours of infection, before proliferation has commenced. Newly activated naive Ag-specific CD8+ T cells produce a rapid pulse of IFN-γ in vivo and begin to accumulate granzyme B and perforin. Later, in vivo cytolytic activity is detectable, coincident with the initiation of cell division. Despite the rapid development of these functional attributes, no antiviral effect was observed early during infection, even when the cells are present in numbers similar to those of virus-specific memory cells. The evolutionary reason for the pulse of IFN-γ synthesis by naive T cells is uncertain, but the lack of antiviral impact suggests that it may be regulatory.