The dura-arachnoid junction is examined in normal animals and in animals subjected to subdural infusion of blood immediately prior to death, simulating acute subdural hemorrhages. The Norwegian landrace pig is used as the experimental animal. Horseradish peroxidase (HRP) has been added to the injected blood to serve as a macromolecular tracer. The material is studied by light and electron microscopy. Special attention is given to the level of the induced subdural cleavage plane, the total distribution of the infused blood, and the natural sites of drainage. The dura-arachnoid junction, identified here as the subdural compartment (the dural border layer of others), consists of an avascular tissue with flake-like, relatively electron-lucent cells stacked upon each other in several layers with narrow intercellular clefts. Under normal conditions there is no evidence of a so-called "subdural space." When under the present experimental conditions bleeding takes place into this cellular tissue, it splits without any particular, predestined cleavage plane, although most often close to the fibrous matter of the dura. The bleeding extends throughout the cerebral and spinal parts of the compartment and also along the spinal nerve roots. Contamination of the subarachnoid space occurs only in some cases subjected to large infusions and apparently only at spinal levels. The HRP diffuses into the dura, but does not traverse the arachnoid barrier layer.