Endothelins (ETs) are a family of potent vasoactive peptides present in the eye. In the rabbit eye, both ET-1 and ET-3 are present in high concentrations in the iris and ciliary body and in lower concentrations elsewhere. Forty-eight hours after injection of 2.5 micrograms of ET-1 and ET-3 into the anterior vitreous of conscious rabbits, intraocular pressure was reduced by 43% and 45%, respectively, and did not return to normal for at least 5 days. The decrease in intraocular pressure was not due to increased aqueous outflow and was not prevented by pretreatment with indomethacin. Injection of either ET-1 or ET-3 (2.5 to 10 micrograms) into the anterior chamber or posterior vitreous cavity resulted in marked vasoconstriction, which was initiated within seconds and persisted for greater than 4 hours. Both endothelins potently constricted the pupil when added to freshly excised rabbit anterior segments. Endothelins are therefore potential participants in the local regulation of intraocular pressure, ocular blood vessel tone, and iris smooth muscle tone and may be important mediators in ocular pathologic conditions. Endothelins at pharmacologic doses may be useful in the control of intraocular pressure or blood flow.