Endothelin-1 (ET-1) is thought to have some effect on the retinal circulation and its autoregulation. The disturbance of ET-1 secretion might contribute to the pathogenesis of retinochoroidal or optic nerve disease. In this study, to determine the effect of ET-1 on the optic nerve head, we observed the optic nerve with a transmission electron microscope and measured element contents in the axon and myelin of the optic nerve head by electron probe X-ray microanalysis. Albino rabbits were given an injection of 0.1 ml (10(-6)M) ET-1 into the middle of the vitreous of one eye (ETs) and 0.1 ml Opeguard-MA into the other eye (controls), and 2 hours after the injection changes in the optic disc were observed. In transmission electron microscopy (n = 3), axon polymorphism and myelin disorder were seen. The X-ray analysis of frozen sections (n = 5) revealed that with ET-1 treatment Ca and Cl concentrations were increased in the axon, and K concentration was decreased significantly in both axon and myelin. These results suggest that the increase of intracellular Ca could elevate the activity of some proteases, which might then cause damage to the optic nerve.