Activation of the hypothalamic-pituitary-adrenal axis (HPA) and sympathetic nervous system by stress has been shown to modulate both innate and adaptive immunity during an experimental influenza A/PR8 viral infection. HPA activation alters levels of glucocorticoids (GC) and opioids which are associated with suppression of lymphoid cellularity and NK activity. These experiments were designed to investigate the role that stress-induced GC and opioids have in modulating NK activity during an influenza viral infection. C57BL/6 mice were treated daily with mifepristone (RU486), a GC receptor antagonist or naltrexone (NTX), a non-specific opioid receptor antagonist. Mice were infected intranasally with A/PR8 virus and underwent daily restraint stress (RST). Phenotypic analysis of splenic cell populations and NK cytotoxicity were assessed at 3 days post-infection. RST of infected mice significantly suppressed splenic CD3(-)DX5+ cellularity and NK cytolytic activity. RU486 administration fully restored splenic NK cellularity but not cytolytic activity. NTX administration restored NK cytolytic activity but not splenic NK cell number. A similar restoration in NK cytolytic activity was observed after administration of beta-funaltrexamine (FNA), a mu-specific opioid receptor antagonist, but not the delta- or kappa-specific opioid receptor antagonists naltrindole or nor-binaltorphimine, respectively. Co-administration of RU486 and NTX restored both NK cellularity and cytolytic activity. The restoration of RST-induced suppression of NK activity by RU486 and NTX or FNA suggests that glucocorticoids modulate NK cellularity and opioids that bind to the mu-opioid receptor modulate NK cytotoxicity during periods of stress and viral infection.