Using morphological, immunocytochemical, and functional parameters we have previously shown that highly purified adult rat microglial cells undergo a process of "activation" when cultured in a serum-containing medium in the absence of added proinflammatory substances or other factors (Slepko and Levi: Glia 16:241-246, 1996). Here we studied the lipopolysaccharide (LPS)-evoked production of two prostanoids, thromboxane A2 (measured as thromboxane B2) (TXB2) and prostaglandin E2 (PGE2), as a function of microglial "activation." LPS induced a greater time- and dose-dependent release of TXB2, compared to PGE2, in the less "activated" cells. Further "activation" led to amplified synthesis of PGE2 and not of TXB2, so that the TXB2/PGE2 ratio changed from 2.2 to 0.25 between the 2nd and 4th day in culture. Western blot experiments showed that the LPS-evoked expression of the inducible form of cyclooxygenase (COX) was markedly higher in cells exhibiting a more "activated" phenotype. The expression of the constitutive isoform of COX was low in all conditions, was slightly greater in more "activated" cells, and was not affected by LPS. Neither progression in microglial "activation" nor LPS treatment enhanced thromboxane synthase activity. We hypothesize that reorientation of prostanoid synthesis toward a major production of PGE2 in the more "activated" cells can be largely attributed to an increased inducibility of cellular COX expression, combined with the inability of thromboxane synthase to cope with the increased availability of the COX product prostaglandin H2 (PGH2), the common precursor of TXA2 and PGE2. In view of the different, and at times opposite, functional activity of TXB2 and PGE2, the described change in prostanoid production pattern may contribute to the role of "activated" microglia in inflammation and host defense.