Sodium valproate, a mood stabilizer, when chronically administered to rats (200 mg/kg i.p. daily for 30 days) significantly reduced the brain protein levels of cyclooxygenase (COX)-1 and COX-2, without altering the mRNA levels of these enzymes. COX activity was decreased, as were the brain concentrations of 11-dehydrothromboxane B2 and prostaglandin E2 (PGE2), metabolites of arachidonic acid (AA) produced via COX. In contrast, the brain protein level of 5-lipoxygenase and the concentration of its AA metabolite leukotriene B4 were unchanged. In view of published evidence that lithium chloride administered chronically to rats, like chronic valproate, reduces AA turnover within brain phospholipids, and that lithium post-transcriptionally down-regulates COX-2 but not COX-1 protein level and enzyme activity, these observations suggest that mood stabilizers generally modulate the release and recycling of AA within brain phospholipids, and the conversion of AA via COX-2 to PGE2 and related eicosanoids. If targeting this part of the 'AA cascade' accounts for their therapeutic action, non-steroidal anti-inflammatory drugs or selective COX-2 inhibitors might prove effective in bipolar disorder.