Intravenous (i.v.) microparticle (MP) injection is a method for selecting broilers with resistance to pulmonary hypertension syndrome (PHS, ascites). In addition to causing pulmonary hypertension, i.v. injected MPs cause a localized inflammation in the lungs of broilers. The objective of this study was to determine the nature of the inflammatory response generated in lungs from MP injected broilers known to be PHS resistant (RES line) or susceptible (SUS line). Specifically, the extent of macrophage infiltration and the expression of chemokines, cytokines, and vasoactive mediators were examined at various times post-MP injection. Included were interleukin-8 (IL-8) and K60; IL-1β, IL-6, IL-4, and interferon-gamma (IFN-γ); endothelin-1 (ET-1), endothelin receptors type A and B (ET_A and ET_B); serotonin transporter (SERT), serotonin receptors: 5-hydroxytryptamine 1A (5-HT_1A), 5-HT_2A, 5-HT_1B, 5-HT_2B; cyclooxygenase-2 (COX-2); hypoxia inducible factor-α (HIF-α); endothelial nitric oxide synthase (eNOS) and inducible NOS (iNOS); Rho kinase (ROCK2); and vascular endothelial growth factor (VEGF). Experiment 1 (Chapter II) showed that macrophage infiltration into the lung increased over time in both the RES and SUS lines. Macrophage infiltration and expression of iNOS was higher in the RES line (P < 0.001). Experiment 2 (Chapter III) revealed a rapid increase in the expression of cytokines and chemokines in lungs from both lines with higher expression in the RES line broilers (P < 0.05). Experiment 3 (Chapter IV) showed that lungs from the SUS line broilers expressed higher levels (P < 0.05) of ET-1, 5-HT_1A, 5-HT_2B and HIF-α, whereas lungs from RES line broilers expressed higher levels (P < 0.05) of eNOS, iNOS, and ET_B. Higher expression of ET-1 and 5-HT_2B in the SUS line may promote vasoconstriction and pulmonary arterial smooth muscle cell (PASMC) proliferation. Higher expression of eNOS and iNOS in the RES line broilers may promote vasodilation and inhibit PASMC proliferation. The observed differential expression of chemokines, cytokines and vasoactive mediators in lungs of MP-injected broilers, with RES line broilers favoring vasodilation and a rapid increase in inflammatory activity and the SUS line favoring vasoconstriction and moderate inflammatory activity, may be key to the PHS-resistance and -susceptibility observed in these lines, respectively.