The effects of different distending pressures on permeability of dog and human arteries to horseradish peroxidase (HRP) were studied. A new catheter was employed to achieve the distention of defined vessel segments to the desired pressure. Normal dog brachial arteries were studied both post mortem and in vivo. Mildly to moderately diseased human coronary arteries were studied post mortem. A predictable linear relationship between pressure and penetration of HRP into the dog arterial media was found, using pressures of 0, 150, 300 and 500 mm Hg. Postmortem vessels were consistently less permeable than those studied in vivo. Full penetration of the media by HRP was achieved by application of 300 mm Hg pressure for 45 sec with the new catheter. When human coronary lesions were examined under these same conditions, plaques were readily demonstrated to be permeable to HRP, even to a depth of many hundreds of micrometer. Thus, penetration of arterial wall thickness by HRP (Mr 40,000 dalton) is related to the distending pressure applied. Human coronary plaques also show ready penetrance by HRP. The new catheter described allows the application of these pressures to defined segments of the arterial tree.