The role of prostanoids in setting the range of autoregulation of retinal blood flow (RBF) and choroidal blood flow (ChBF) in the newborn was assessed. The RBF, ChBF, and arterial and cerebral sinus concentrations of PGE, PGF2 alpha, 6-keto-PGF1 alpha and TXB2 were measured over a wide range of mean systemic blood pressure (blood pressure (BP): 17-117 mm Hg) in newborn piglets treated with ibuprofen (30 mg/kg iv) or its vehicle (n= 8, in each group). Hypertension and hypotension were induced 80 min apart on each animal, by inflating balloon-tipped catheters placed at the aortic isthmus and root, respectively. Blood flow and prostanoid concentrations were measured 20 min before (basal) and during the induced changes in BP. In vehicle-treated piglets, RBF did not change with BP between 50 and 90 mm Hg (r= 0.33, P= 0.27), but changed as a function of BP beyond this range (tau= 0.52, P less than 0.01); ChBF increased with BP throughout the range studied (17-117 mm Hg; tau= 0.89, P less than 0.001). The relationship between O2 delivery to the retina and choroid and BP (tau greater than 0.43, P less than 0.01) was similar to that seen between RBF and ChBF with BP. The concentration of all prostanoids increased when BP was reduced to less than 50 mm Hg. When BP was raised to more than 90 mm Hg, prostaglandin concentrations increased, and those of TXB2 did not change. Ibuprofen treatment reduced the basal concentrations of all prostanoids to nearly undetectable levels and prevented their changes during hypotension and hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)