In the rabbit iris sphincter muscle, transmural electrical stimulation produces a substance P-ergic response, in addition to a cholinergic one. The substance P-ergic response was selectively attenuated or abolished by (D-Pro2, D-Trp7,9) substance P or trigeminal denervation. In the present work, the effects of Met-enkephalin on these two neurogenic responses were examined. Met-enkephalin (10(-7) to 10(-5) M) inhibited the substance P-ergic and cholinergic responses, in a dose-dependent manner and the inhibitory action of Met-enkephalin was greater in the case of low frequency stimulation. These inhibitory effects of Met-enkephalin were antagonized by naloxone (10(-5) M). On the other hand, Met-enkephalin had no effects on the responses to exogenously applied substances P and carbachol. These results suggest that the trigeminal, substance P-ergic neurons as well as cholinergic nerves have opiate receptors and that enkephalin inhibits the neuronal responses presynaptically, probably the release of substance P or acetylcholine, through such opiate receptors.