Nitric oxide (NO), a free radical produced by the enzyme NO synthase (NOS), is a potent antiviral agent in addition to having immune regulating functions. Recently, it was reported that chickens resistant (N2a, MHC: B²¹B²¹) to the development of Marek's disease (MD) had a greater potential to produce NO than MD-susceptible chickens (P2a, MHC: B¹⁹B¹⁹). This difference was shown by measuring NO levels in chick embryo fibroblast cultures obtained from these chickens after treatment with lipopolysaccharide and recombinant chicken interferon-gamma (IFN-γ). To extend these results, the levels of NO in blood plasma from N2a and P2a chickens inoculated with the nonattenuated JM-16 strain of MD virus (MDV) were examined. In four out of five experiments, N2a chickens had increased NO levels at 7 days postinoculation (DPI). In contrast, P2a chickens challenged with JM-16 had a significant increase in NO in only one of four experiments, and in that experiment the increase was delayed (10 DPI) compared with N2a chickens. Attenuation abrogated MDV-induced NO in chickens. Inoculation with MDV strains ranging from mild to very virulent plus showed that the more virulent strains induced the highest level of NO in blood plasma, suggesting a role of NO in the pathogenesis of MD with more virulent strains. On the basis of quantitative real-time reverse transcription–polymerase chain reaction (RT-PCR) assays for analysis of mRNA expression, IFN-γ does not appear to be the primary inducer of inducible (i)NOS gene expression during MDV infection. iNOS gene expression and NO production are mediated during the cytolytic phase of MDV infection on the basis of real-time RT-PCR assays with primers specific for glycoprotein B, a late gene expressed only during the cytolytic phase of MDV infection. These findings implicate NO as a factor potentially involved in increasing virulence of MDV, possibly through immune suppression.