Using horse radish peroxidase as a tracer for fluid flow, rapid diffusion was demonstrated between sinusoidal lumina and the interhepatocytic spaces of the murine liver. The marker passed rapidly between adjacent sinusoidal cells and filled the space of Disse. Appreciable amounts of peroxidase were also seen in pinocytic and phagocytic vacuoles within sinusoidal cells. Many junctions in the wall of bile canaliculi impeded the passage of the marker, but in some sites this hindrance did not occur. This provides a path for the direct exchange of material between vascular and biliary contents. Such a mechanism may be physiologically controlled. Junctions in bile ductules prevent diffusion of horse radish peroxidase.