Delivery of DNA to the central nervous system via the cerebral circulation for a possible treatment of disorders affecting the entire brain is prevented by the blood-brain barrier (BBB). Although BBB disruption (BBBD) can be obtained by intracarotid infusion of hyperosmolar solutions, the size of the resulting interendothelial gaps does not allow extravasation of cDNA transgenes. We have assessed brain delivery of DNA complexed with the DNA-compacting, cationic proteins poly-L-lysine (PLL) and salmon sperm protamine (SSP) after osmotic BBBD in 26 rats, including 7 controls. The 5kb plasmid, pGL encoding A. victoria green fluorescent protein (GFP) was reacted with PLL or SSP under carefully controlled conditions to obtain DNA-protein complexes 10-20 nm in diameter, as assessed by electron microscopy. The DNA suspensions were infused into the common carotid artery of rats after unilateral BBBD. This was obtained by ligation of the ipsilateral occipital and pterigopalatine arteries, temporary clamping of the external and common carotid arteries distal and proximal to an indwelling catheter, and infusion of 1.7 molal arabinose (0.1 ml in 10 s). The presence of plasmid DNA in homogenates of brain hemispheres and cerebellum from 16 animals sacrificed and saline-perfused at 2 hours was assessed by the polymerase chain reaction(PCR). PCR product was detected in 15 of these 16 animals, usually with an intensity corresponding to the degree of BBB permeabilization, as judged by Evans blue-albumin (EBA) extravasation. Separation of DNA from endothelial cells and plasma membranes by a modified capillary depletion method supported the contention that most of the plasmid DNA was present in the brain parenchyma. In fact, little or no PCR product was detected in the brain of 4 animals infused with compacted DNA in the absence of BBBD, indicating minimal binding or internalization of DNA-protein complexes by vascular endothelial cells. Predictably, no PCR product was detectable after BBBD and infusion of uncomplexed DNA. Neural cell transduction by PLL-cDNA resulting in GFP expression was assessed by fluorescence- and confocal microscopy in 2 rats sacrificed 5 days after treatment. Paravascular and parenchymal cells exhibiting green cytoplasmic fluorescence were visible in diverse brain regions, usually coincident with red-fluorescing, extravasated EBA. These initial data indicate that transduction of neural cells can be obtained by delivery of compacted DNA-protein complexes to the brain parenchyma across a reversibly permeabilized BBB.