Myointimal hyperplasia is the condition usually responsible for recurrent stenosis (restenosis) after endarterectomy, bypass grafting and angioplasty. Its cause is still not known. The present study examined whether inhibition of thrombin by tissue plasminogen activator (r-TPA) or polyethylene glycol recombinant hirudin (PEG-hirudin) could reduce restenosis in an animal model. Restenosis was induced in 20 cholesterol-fed rabbits. The right carotid artery underwent a double-balloon injury while left carotid artery acted as a control. Recombinant tissue plasminogen activator (1 mg kg(-1) s.c.) and PEG-hirudin (0.7 mg kg(-1) s.c.) were given subcutaneously with normal saline acting as a control. Blood levels of PEG-hirudin were measured by both ELISA and an Ecarin (activity) assay. Vessel dimensions were measured in histological sections, obtained from perfusion-fixed tissue, using computerised planimetry. The model reproduced many of the histological changes found in human restenosis, such as intramural thrombus, rupture of the elastic lamina, macrophage infiltration and smooth muscle migration. Reinjury caused an almost three-fold reduction in the area of the lumen (median 0.25 mm(2)) compared with uninjured vessels (median 0.72 mm(2)). The mean plasma levels of PEG-hirudin and r-tPA achieved were 291 ng/ml (S.E.M. 28 ng/ml) and 34 IU/ml (S.E.M. 12 IU/ml), respectively. PEG-hirudin significantly inhibited the effect of balloon injury on luminal area compared with saline-treated controls (0.21 versus 0.44 mm(2), respectively, P<0.05). Recombinant tPA also had a similar inhibitory affect, but this did not reach statistical significance (0.16 versus 0.44 mm(2), respectively, P>0.05). The magnitude of luminal narrowing was significantly reduced by subcutaneous injection of PEG-hirudin. Further studies are required to determine whether this effect can be enhanced by other antithrombins or improved methods of delivery.