Two skulls of Late Cretaceous multituberculates from Mongolia, Nemegtbaatar gobiensis and Chulsanbaatar vulgaris, were serially sectioned on a Jung microtome at thicknesses respectively of 25 and 20 <latex>$\mu$</latex> m. A wax model of the endocranial cavity, with casts of certain vessels and nerves, was made for Nemegtbaatar. On the basis of the sections, model, various entire skulls from Mongolia and isolated Late Cretaceous multituberculate petrosals from Montana, U.S.A., a reconstruction of the vascular system was made. The internal carotid artery entered the pituitary fossa laterally at the junction of petrosal, basisphenoid and probably alisphenoid and pterygoid. A stapedial artery was present, possibly with a meningeal branch entering the skull through the `hiatus Fallopii'. In some genera a canal ran from the facial sulcus to open in front of the mandibular nerve foramina outside the semilunar fossa, possibly transmitting the maxillary artery. In addition to undoubted arteries two other systems of cranial vessels were apparent: a dural sinus system and an orbito-temporal system. The dural sinus system included: sagittal sinus, transverse sinus, sigmoid sinus, prootic sinus, tentorial sinus; it received a tributary glenoprootic vein, which passed medially through a canal to join the ventral part of the prootic vein. The primary head vein passed from the cavum epiptericum through a post-trigeminal canal into the facial sulcus, received the prootic vein here and probably left the sulcus as the stylomastoid vein before joining the jugular system. The orbito-temporal system consisted of vessels that ran from the post-temporal fossa, through an ascending canal running between the anterior lamina of the petrosal and the squamosal, to enter the cranial cavity. Then it passed across the prootic sinus and bifurcated at the postorbital process with an intracranial branch running anteriorly and an extracranial branch running in the postorbital groove to the ethmoid foramen. In forms with a large post-temporal fossa the vessels entered the post-temporal recess of the subarcuate fossa to run to the ascending canal. In forms with a small post-temporal fossa the vessels ran to the ascending canal without entering the subarcuate fossa, and had a major communication with either the stylomastoid or the postglenoid foramen. This route shows considerable similarity to that of the arteria diploetica magna of Tachyglossus, and examination of the embryos of monotremes, marsupials and placentals suggests that an artery, not a vein, should be considered as the major vessel primarily associated with this orbitotemporal system, although the presence of companion veins can be expected. Because a very similar system occurs also in triconodonts, tritylodonts and cynodonts, although with larger portions extracranial, the attention of students of those groups is called to the possibility that this orbitotemporal or `sinus canal' system may be primarily arterial rather than, as hitherto accepted, venous. The cavum epiptericum was completely floored and was separated from the cranial cavity by a bony wall consisting of the orbitosphenoid with a persistent pila antotica. The optic foramen may have pierced this medial wall. The semilunar ganglion was very large, the mandibular nerve leaving the skull by two main foramina, though a small foramen piercing the anterior part of the anterior lamina may have been for a deep temporal branch of the nerve. The sphenorbital fissure is hardly visible in lateral view, forming the anterior opening of a large cavum epiptericum lying between the anterior lamina and the posterior part of the orbitosphenoid. It is suggested that the bone previously referred to as tabular may be the mastoid part of the petrosal because it included the semicircular canal. In some genera the occipital condyles are hollow, the cavity probably acting as an accessory tympanic air sinus. The endocranial cast of Nemegtbaatar reconstructed here differs in proportions from the previously known endocast of Chulsanbaatar, and is characterized by large olfactory bulbs, shallow telencephalon with cerebral hemispheres extensive in dorsal aspect and concave latero-ventrally, deep rhombencephalon, no midbrain exposure on the dorsal surface, large central lobe of cerebellum, no obvious cerebellar hemispheres and relatively very large paraflocculi. The pons is situated caudal to the emergence of the trigeminal nerve, as in monotremes. A glossary is given of the osteological, vascular and neuroanatomical terms used in this paper.