Bacterial collagenase causes hemorrhagic necrosis of brain. We studied the enzyme's effect on blood-brain barrier (BBB) permeability and extracellular matrix (ECM) structure by radiolabeled tracers and electron microscopy. Adult rats had intracerebral injection of bacterial collagenase. Brain uptake from blood of [14C]sucrose was measured in 24 rats 0.5 h to 14 days after injection. 12 rats had ultrastructural studies 1 h after collagenase injection. Brain uptake of [14C]sucrose is maximally increased at 0.5 h, remaining significantly increased for 7 days. Ultrastructurally, some vessels had widening of basal lamina while others had severe disruption of basal lamina with stretching of endothelial cells. We conclude that bacterial collagenase disrupts ECM and opens BBB.