The peptide endothelin (human, porcine) was investigated for effects on basal muscle tone and on responses to transmural nerve stimulation in a series of smooth muscle preparations, as well as in guinea-pig atrium and rat and guinea-pig diaphragm. Endothelin lacked effect on basal tone or on spontaneous and electrically driven contractions in skeletal and atrial muscle. It contracted guinea-pig ileum, pulmonary and femoral arteries, rat anococcygeus, vas deferens and urinary bladder and rabbit taenia coli, whereas guinea-pig taenia was relaxed. Guinea-pig urinary bladder and vas deferens and rabbit iris sphincter were unaffected up to 3 x 10(-8) M. Endothelin thus has a unique pattern of smooth muscle effects, exhibiting mostly contractile but also relaxing effects. Endothelin modified contractile responses to transmural nerve stimulation, yielding marked and persistent enhancement, in guinea-pig and rat vas deferens, and enhancement also in guinea-pig pulmonary artery. In guinea-pig and rat vas deferens the response to exogenous ATP was increased by endothelin, thus suggesting a strong post-junctional enhancement of neurotransmission. In guinea-pig ileum nerve-induced responses were inhibited by endothelin, whereas exogeneous acetylcholine was enhanced, an effect suggesting a simultaneous pre-junctional inhibition and post-junctional enhancement. The Ca2+ channel blocker felodipine counteracted the stimulatory effects of endothelin on tone and transmurally induced contractions. Tachyphylaxis to endothelin action was sometimes evident, but the anococcygeus being less prone to this might be useful for studies on endothelin antagonism. Endothelin thus has prominent post-junctional, and also probably pre-junctional, effects, lending further support for a distinct biological role of this peptide.