Nitric oxide (NO) is a labile inorganic free radical produced by NO synthase from the substrate l-arginine in various cells and tissues including endothelial cells. A substantial elevation of nitrite levels indicative of NO production occurred in cultures of Cowdria ruminantium-infected bovine pulmonary endothelial cells (BPEC) incubated in medium alone. Exposure of the infected cultures to recombinant bovine gamma interferon (BorIFN-gamma) resulted in more rapid production of NO, reduced viability of C. ruminantium, and induction of endothelial cell death. Significant inhibition of NO production was noted after addition of the NO synthase inhibitor N-monomethyl-l-arginine (l-NMMA), indicating that the increase in production occurred via the inducible NO synthase pathway. Reduction in the infectivity of C. ruminantium elementary bodies (EBs) occurred in a dose-dependent manner after incubation with the NO donor molecule S-nitroso-N-acetyl-dl-penicillamine (SNAP) prior to infection of endothelial cells. The level of infection in cultures maintained in SNAP was reduced in a dose-dependent manner with significant negative correlation between the final level of infection on day 7 and the level of SNAP (r = -0.96). It was established that pretreatment and cultivation of C. ruminantium EBs with the NO donor molecule SNAP reduced infectivity to cultures and viability of EBs with the implication that release of NO in vivo following infection of endothelial cells may have an effect upon the multiplication of the agent in the host animal and may be involved in the pathogenesis of heartwater through the effect of this molecule upon circulation.