This study has examined the localisation and receptor-binding of the endothelins in retina and choroid of human and rat origin. Immunoreactivity to anti-ET1 and anti-ET3 was investigated in trypsin digests, frozen sections and ultrathin sections using immunocytochemistry and immunogold labelling techniques. In addition, receptor binding of125I-ET1 and125I-ET3 was visualised and quantified using autoradiography and image analysis. Intense immunoreactivity to anti-ET1 and anti-ET3 was observed in the photoreceptor inner segments and in the outer plexiform layer (OPL) of human and rat retina. Ultrastructural localisation using immunogold labelling confirmed the presence of ET1 and ET3 in the photoreceptor cells. In retinal vascular digests, ET1 was visualised in the arteries, arterioles and at the pre-arteriolar sphincters, however, immunoreactivity to anti-ET3 was absent in the retinal vasculature. Both ETAand ETB-type receptor binding sites to125I-ET1 and125I-ET3 were detected in the vascular smooth muscle of choroidal and retinal vessels with the former being predominant. Extravascular binding sites of the ETB-type were found in the ganglion cell layer.