Thermal injury increases the capacity of macrophages (Mφ) to produce various inflammatory mediators, (i.e., Mφ hyperactivity), which is believed to be involved in the development of subsequent immunosuppression, sepsis, and multiple organ failure. The signal transduction pathways involved in the expression of Mφ hyperactivity post-burn, however, remain to be clearly elucidated. To study this C57BL/6 female mice were subjected to a 25% TBSA burn and splenic Mφs were isolated 7 days later. LPS-stimulated inflammatory mediator production and MAPK expression (P38 ERK 1/2 and JNK) were determined. Burn injury increased LPS-induced P38 MAPK, suppressed JNK activation and ERK 1/2 activation was unaltered. These changes in MAPK activation were paralleled by the increased production of PGE2, TNF-α, IL-1β, IL-6, and IL-10. Differential sensitivity to the inhibition of the MAPK pathways was observed with regard to the mediator evaluated and the presence or absence of burn injury. In general cytokine production in the burn group was in part resistant to the inhibition of a single MAPK pathway as compared with shams. Thus, burn injury increases cross-talk between the MAPKs pathways, suggesting that alterations MAPK activation and signal transduction contribute to the development Mφ hyperactivity post-injury. © 2004 Wiley-Liss, Inc.