To clarify the role played by prostaglandins in acute brain inflammation we studied the expression of the key enzyme in their formation, cyclooxygenase-2 (COX-2), following microinjection of bacterial endotoxin (LPS), interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma), in the rat dorsal hippocampus. In spite of the extensive astrocyte and microglial reaction, at 24 hours after LPS injection COX-2 immunoreactivity (COX-2-ir) was exclusively associated with infiltrating neutrophils and with perivascular cells of blood vessels in the area surrounding the injection site. Microinjection of IFN-gamma did not alter COX-2-ir, whereas TNF-alpha or IL-1beta injection induced a moderate COX-2-ir in the perivascular cells of a few blood vessels close to the injection site, and in very few of the infiltrating neutrophils. When IL-1beta, but not TNF-alpha or INF-gamma, was injected in combination with LPS, a strong COX-2-ir was associated with the perivascular cells of most blood vessels in the injected hemisphere and of several of those in the uninjected hemisphere. In addition, COX-2-ir was detected in neutrophils and in several parenchymal cells surrounding the injection site. The parenchymal and perivascular COX-2-positive cells showed a microglia/macrophage-like morphology, as compared with the GSI-B4 isolectin and ED-1 staining, specific for macrophages. Since the constitutive neuronal COX-2 was not affected by any of the conditions studied, the macrophage-like cells found in the perivascular region and in the parenchyma may represent the main source of prostaglandins during focal inflammatory responses in the brain.