Many chemical mediators regulate neutrophil recruitment to inflammatory sites. Although the actions of each chemical mediator have been demonstrated with neutrophils in vitro, how such chemical mediators act cooperatively or counteractively in vivo remains largely unknown. Here, by in vivo two-photon excitation microscopy with transgenic mice expressing biosensors based on Frster resonance energy transfer, we time-lapse-imaged the activities of extracellular signal-regulated kinase (ERK) and protein kinase A (PKA) in neutrophils in inflamed intestinal tissue. ERK activity in neutrophils rapidly increased during spreading on the endothelial cells and showed positive correlation with the migration velocity on endothelial cells or in interstitial tissue. Meanwhile, in the neutrophils migrating in the interstitial tissue, high PKA activity correlated negatively with migration velocity. In contradiction to previous in vitro studies that showed ERK activation by prostaglandin E-2 (PGE(2)) engagement with prostaglandin receptor EP4, intravenous administration of EP4 agonist activated PKA, inhibited ERK, and suppressed migration of neutrophils. The opposite results were obtained using nonsteroidal antiinflammatory drugs (NSAIDs). Therefore, NSAID-induced enteritis may be caused at least partially by the inhibition of EP4 receptor signaling of neutrophils. Our results demonstrate that ERK positively regulates the neutrophil recruitment cascade by promoting adhesion and migration steps.