Levels of free arachidonic acid and of prostaglandin F2alpha and E2 have been measured in both brain cortex and cerebellum of rats killed by focussed microwave irradiation, and after decapitation followed by ischemia. The same parameters were studied during incubation assays. It was found that: a) after ischemia levels of both free arachidonic acid and of prostaglandins in cerebellum are lower than in brain cortex, b) formation of prostaglandins from endogenous precursor in incubated cortex is higher than in cerebellum, c) release of free arachidonic acid occurs mainly during the time interval between the sacrifice of the animals and the beginning of the incubation, whereas prostaglandins are formed mainly during the incubation assay. The correlation between release of free arachidonic acid and prostaglandin formation is discussed.