Cultured murine cerebral microvessel endothelia produce predominantly prostacyclin and prostaglandin (PG) E2 when exposed to trace amounts of arachidonic acid. At higher concentrations of arachidonate or with ionophore A23187, they produce more PGE2 than prostacyclin and additionally make PGF2 alpha and small amounts of eicosanoids comigrating with hydroxylated derivatives of arachidonate. Endothelia grown on micropore filters release prostaglandins from both apical and basal surfaces; however, the ratio of basal to apical release is as high as 4:1. This polarity suggests that cerebral endothelial prostaglandins can interact with neighboring cells of the vessel wall and brain parenchyma, where they may play important roles in the control of cerebrovascular tone and neuroglial function. These eicosanoids also are produced by mouse brain microvessels, but the major microvessel product is 12-hydroxyeicosatetraenoic acid. This suggests that 12-hydroxyeicosatetraenoic acid may be abundant in brain injuries after arachidonate release. Cultured cerebral endothelia rapidly convert 12-hydroxyeicosatetraenoic to more polar metabolites and thus may prevent the accumulation of this potentially deleterious hydroxyacid.