Male Sprague-Dawley rats were treated with D-penicillamine (D-pen) 500 mg/kg/day for 10 or 42 days. Pair fed rats served as controls. Changes in aortic morphology were examined by light- and transmission-electron microscopy (TEM). In addition, the endothelial permeability and the penetration through the aortic wall of albumin were studied 10 minutes, 24 and 48 hours after i. v. injection of human serum 131I-albumin (131I-HSA). TEM revealed extensive elastolysis in the arterial wall of D-pen-treated rats, consistent with an inhibitory effect on crosslink formation. In experimental animals excess deposition of collagen and glycoaminoglycans was observed in the subendothelial and medial layer of the aortic wall, together with prominent basal membrane substance around aortic smooth muscle cells. The aorta/serum-ratio and the radioactive build-up 24 and 48 hours after injection of 131I-HSA was reduced in animals treated with D-pen for 42 days, indicating an impeded transmural transport of tracer which may be caused by a steric exclusion effect of abundant hyaluronate. The endothelial ultrastructure was unaffected by D-pen, and no differences in aortic 131I-HSA radioactivity or aorta/serum-ratio were recorded between experimental and control groups 10 minutes after tracer injection, indicating that the permeability of the endothelial barrier to albumin remained unaffected by D-pen treatment. These observations support the hypothesis that treatment with high doses of D-pen may induce a fibroproliferative response in rat aorta, possibly by an inhibitory effect on the cross-linking of collagen and elastin.