Background: Macrophages have important role in defense against Herpes Simplex Virus type-1 (HSV-1). The present study was performed to determine the viability and nitric oxide (NO) production by HSV-1 infected mouse peritoneal macrophages (HIM). Method: The viability of macrophages was evaluated using MTT reduction assay and the production of nitrite using Griess method. Results: The ability of infected macrophages to reduce Tetrazolium (MTT) was diminished at virus to cell ratios of multiplicity of infection (MOI) of one, three and 10; but not at 0.01and 0.1. Induction and inhibition of NO production by HIM were MOI dependent. The basal NO production by these cells was inhibited at MOI of three and ten. In contrast virus to cell ratios of 0.01 and 0.1 induced low but significant enhancement in NO production. The inability of HIM to reduce MTT at MOI of three was significant after 12-hrs and inhibition of NO production was initiated between 12-20 hours after infection. Conclusion: High doses of HSV-1 seem to decrease the normal activity of macrophages by inhibiting the production of nitric oxide.