Endothelin (ET)-1 (10 nM) is about six times more effective than ET-3 in contracting the isolated iris sphincter muscle; the ET-1-induced contraction is insensitive to indomethacin treatment. The effect of ET-2 is intermediatory between ET-1 and ET-3 in contracting the muscle. The relative potency of the ETs to stimulate inositol phosphates (InsPs) in the iris-ciliary processes is ET-1 > ET-2 > ET-3, with ET-1 about six times more potent than ET-3; these effects are also insensitive to indomethacin. Studies utilizing the polymerase chain reaction (PCR) show that ETB receptors are present. Although no evidence could be found for the occurrence of ETA receptors, their presence cannot be excluded. These results suggest that the stimulation of InsPs and contraction of the iris sphincter muscle by ET is mediated by ETB receptors and that products generated via activation of phospholipase A2 are not indirectly involved in the observed responses. However, another type of ET receptor is indicated by the finding that ET-1 reduces the forskolin-elevated cAMP levels in the iris-ciliary epithelium. Autoradiographic results show that specific [125I]ET-1 binding sites are associated with the iris, ciliary processes and the corneal endothelium. As in the iris-ciliary process tissues, ET-1 is the most effective of the three ETs stimulating InsPs in the cornea, although statistically the differences were insignificant. Moreover, ET-1 was found to have no effect on the forskolin-elevated cAMP levels in the cornea. Whether these results reflect true differences between the ET receptors in the iris-ciliary processes and corneal endothelium remains to be established.