In primate eyes the injection of Endothelin (ET) 1 into the anterior chamber lowers the intraocular pressure by increasing outflow facility. A possible source for aqueous ET is the ciliary epithelium, since virus transformed human nonpigmented ciliary epithelial cells secrete ET into the medium and immunohistochemically stain with antibodies against ET 1.This study demonstrates that human ciliary epithelium exhibits positive endothelin-like immunoreactivity with antibodies against ET 1. There are, however, regional differences with regard to the staining intensity. Pronouced staining was observed in the nonpigmented and pigmented epithelium at the crests of the pars plicata. In the valleys of the pars plicata and in the pars plana no immunolabeling was found.