Responses of pial arterioles to topically applied arachidonic acid, conversion of exogenous arachidonic acid to prostanoids, and pial arteriolar dilation to hypercapnia were examined before and at progressive times after treatment with indomethacin (5 mg/kg i.v.) in chloralose-anesthetized newborn pigs with closed cranial windows. Before treatment with indomethacin, arachidonic acid and hypercapnia dilated pial arterioles and increased cortical periarachnoid cerebrospinal fluid concentrations of 6-keto-prostaglandin (PG) F1 alpha and PGE2. One h after indomethacin treatment, the dilations and prostanoid synthesis were blocked. By 2 h after indomethacin treatment, hypercapnia produced significant dilation of pial arterioles, and dilation to both stimuli had returned to preindomethacin levels by 3 h. Inhibition of conversion of exogenous arachidonic acid to prostanoids as monitored by increases in 6-keto-PGF1 alpha and PGE2 in cerebrospinal fluid under the window also was reversed by 3 h after treatment with indomethacin. Repeated indomethacin treatment again blocked dilations and conversion of arachidonic acid to prostanoids on the brain surface. The possibility of short duration of vascular effectiveness of indomethacin when it is administered systemically needs to be considered, both when it is used as a probe for understanding contributions of PGH synthase products to control of cerebral circulation and when it is used therapeutically in attempts to alter the newborn cerebral circulation.