The capacity for biosynthesis of prostaglandin F2alpha (PGF2alpha) and prostaglandin E2 (PGE2) from endogenous precursors by brain tissue slices and homogenates was measured by a gas chromatography - mass fragmentography method using deuterated prostaglandins as internal standards. Mean biosynthesis in rat cerebral cortex slices incubated for 60 min was 60.2 ng PGF2alpha and 17.4 ng PGE2 per 100 mg of tissue. The corresponding values for homogenates were 78.1 ng and 28.9 ng. Synthetic capacity of cat cerebral cortex was considerably greater but that of human tissue was smaller than that found in rat brain. Cat cerebellum in contrast to other regions synthesized more PGE2 than PGF2alpha. The time-course of prostaglandin formation in slices was linear for the initial 60 min. Catabolism in cerebral tissues was found to be very small. Prostaglandins formed or added to the incubation media distributed between tissue and medium in a manner indicating some specific binding as well as nonspecific solubilization in tissue lipids. Norepinephrine, 3,4-L-dihydroxyphenylalanine, dopamine, adrenochrome and apomorphine greatly stimulated PGF2alpha formation probably through a nonenzymatic reduction of endoperoxides. Norepinephrine added to homogenates appeared to stabilize the fatty acid cyclo-oxygenase preventing it from inactivation. Indomethacin and Ketoprofen were potent inhibitors of biosynthesis. Paracetamol was found to be a less potent synthetase inhibitor than aspirin. The biosynthetic capacity of brain tissue in vitro appears to be orders of magnitude more than that of normal brain in situ.