The contractile activity of endothelins (ETs) and their effect on twitch contraction evoked by electrical field stimulation were studied in the isolated rabbit iris sphincter and dilator muscles using the isometric tension recording method. ET-1, ET-2 and ET-3 caused slow contractile response of the sphincter as well as the dilator muscles, with sphincter muscle EC50values of 37.3, 58.0 and 84.3 nM, and dilator muscle values of 31.7, 69.0 and 224.4 nM, respectively. These contractile responses were not affected by atropine (1μM), prazosin (1μM) or indomethacin (1μM).ET-1 and ET-3 slightly but significantly inhibited the fast cholinergic component of twitch contraction of the sphincter muscle evoked by field stimulation, without affecting the response to carbachol. However, ET-1 did not affect the slow substance P-ergic component of twitch contraction of this preparation. In the dilator muscle, ET-1 and ET-3 had little effect on the contraction induced by phenylephrine but reduced the amplitude of the twitch contraction evoked by the field stimulation. The inhibitory effect of ETs on the dilator muscle was antagonized by pretreatment with BQ-123 (100 nM), which is a selective ETA receptor antagonist. Pretreatment with indomethacin (1μM) also attenuated the inhibitory effect of ETs on the twitch contraction.These results suggest that ETs may have two sites of action, pre- and post-synaptic effects, on the rabbit iris sphincter muscle as well as the dilator muscle. All three ET isopeptides may have direct contractile activity on the smooth muscles of the sphincter as well as the dilator without mediation by prostaglandins or neuronal mechanisms. Furthermore, ETs may act on nerve endings to inhibit adrenergic and cholinergic neurotransmission in the iris dilator and sphincter, respectively.