A study has been made of the arterial and venous circulation of the preoptic region in the rat, using the double Indian ink technique. A detailed description is offered on supply of the nucleus preopticus medialis, lateralis, suprachiasmatis and periventricularis, as well as of the nucleus interstitialis striae criminalis and of the preoptic portion of the medical forebrain bundle. The arteries of the region emerge from the anterior communicans artery and from the anterior cerebral artery. The blood of the region is collected by the anterior cerebral artery. Veins collected by the v. cerebri magna, are also running dorsalwards from the nucleus interstitialis striae terminalis. The blood supply of the organon vasculosum laminae terminalis is described with special regard to its vascular connection with the preoptic nuclei.