Free radicals and some free fatty acids, such as arachidonic acid metabolites, have been hypothesized to be contributors to secondary damage to the spinal cord upon injury. These two types of species may form a feedback loop in which generation of one type leads to formation of the other. In this study, to determine whether hydroxyl radical causes generation of arachidonic acid metabolites in vivo, we generated hydroxyl radical, a most reactive oxygen radical, in the rat spinal cord and measured resulting changes in levels of prostaglandin F2 alpha, an arachidonic acid metabolite that rises following traumatic injury. The hydroxyl radical was generated in the rat spinal cord by administering H2O2 through one microdialysis fiber and FeCl2/EDTA through a parallel fiber. The prostaglandin F2 alpha in the collected microdialysates was measured by HPLC as its 3-bromomethyl-6,7-dimethoxy-1-methyl-2-(1H)-quinoxalinone derivative. Prostaglandin F2 alpha dramatically increased in response to hydroxyl radical generation, but declined substantially after 3 h of exposure. Prostaglandin F2 alpha was undetectable when either H2O2 or FeCl2/EDTA was administered alone in control experiments, demonstrating that its formation was caused by generated hydroxyl radical.