The influence of endothelium on the neurogenic component of ouabain-induced contractions in isolated perfused guinea pig carotid arteries was analyzed. Ouabain (0.1 mumol/L to 0.1 mmol/L) evoked concentration-dependent increases of perfusion pressure. Phentolamine (0.3 to 10 mumol/L) and prazosin (30 nmol/L to 10 mumol/L) (nonselective antagonist of alpha-adrenergic receptors and selective antagonist of alpha 1-adrenergic receptors, respectively) induced a concentration-dependent relaxation in segments precontracted with ouabain (0.1 mmol/L). When the arteries were preincubated with those blockers (both at 3 mumol/L) or the animals were pretreated with reserpine, the contractions to the glycoside were diminished, indicating that they are partially mediated by norepinephrine release from adrenergic nerve endings. De-endothelialization abolished the effect of adrenergic blockade on ouabain-induced contractions. On the other hand, de-endothelialization did not modify significantly the effect of the adrenergic blockade on norepinephrine-induced contractions. The nitric oxide blocker oxyhemoglobin, at concentrations (10 mumol/L) that abolished endothelium-dependent relaxations induced by 3 mumol/L acetylcholine; or the cyclooxygenase blocker indomethacin (10 mumol/L) did not modify the relaxation caused by phentolamine. In bioassay experiments, 30 mumol/L phentolamine induced a relaxation on the ouabain-elicited contraction only when the glycoside was added through a donor segment with endothelium. Ouabain-induced tritiated norepinephrine release was significantly reduced by the removal of endothelium but not by 1 mumol/L oxyhemoglobin or 1 mumol/L indomethacin. These results suggest that the endothelium modulates the neurogenic component involved in contractions evoked by the glycoside by a diffusible factor (or factors) whose nature is unknown; however, the factor is neither nitric oxide nor a cyclooxygenase-related compound.