We studied the short- and long-term histological responses induced by conventional and modified electronically detachable coils (GDCs) in experimental aneurysms. Eighteen carotid bifurcation aneurysms were produced microsurgically in rabbits. Six animals each were treated either with conventional or with GDCs coated with a mixture of tissue-thromboplastin to enhance intra-aneurysmal thrombus formation and of plasminogen activator inhibitor type-1 (PAI-1) in inhibit intra-aneurysmal clot fibrinolysis. Six served as untreated controls. Follow-up angiograms were obtained immediately and at 3, 6, 9, 12, 17, and 24 weeks after embolization prior to sacrifice of the animals. All aneurysms were studied macroscopically and histopathologically with the coils in situ. Five of six control aneurysms remained patent. Endovascular occlusion rates between > 90% and 100% were achieved in nine of twelve coiled aneurysms. Follow-up angiography demonstrated recanalization and coil compaction in 5 of them. Gross and microscopic histopathological examination revealed a membrane covering the orifice, intra-aneurysmal scar formation, and development of a neo-intima in both treatment groups at 17 and 24 weeks postembolization. The granulation tissue response appeared to be equally distributed in aneurysms treated with either uncoated or coated coils. Further quantitative morphometric studies are needed to prove if a thrombogenic/antithrombolytic coil-coating might be of value in providing a more enduring anatomic result after GDC-treatment of human brain aneurysms.