Abstract The synthesis of prostacyclin and prostaglandins was examined in isolated blood-free brain capillaries of guinea-pigs and rats using 1- 14C-arachidonic acid as a precursor. The main prostaglandins synthesized by guinea-pig microvessels were prostaglandin D 2 and prostaglandin E 2. Substantially less prostaglandin F 2 α or the prostacyclin stable metabolite, 6-oxo-prostaglandin F 1 α was synthesized. Rat capillary prostaglandin distribution differed substantially from that of the guinea-pigs although the principle prostaglandin was also PGD 2. Total prostaglandin conversion was greater in guinea-pig capillaries than in the rat. Norepinephrine stimulated the prostaglandin forming capacity of blood free cerebral microvasculature of guinea-pigs. Prostacyclin and prostaglandins could be involved in the activity dependent regulation of regional cerebral blood flow and permeability.