The actions of opioid receptor agonists and antagonists were studied in isolated rat aortic strips. Morphine (10(-7)-10(-6) M) had no contractile effect on resting strips but when added during the relaxation of the contractions induced by 10(-9) M noradrenaline, it induced a contractile response which was blocked by naloxone. The selective mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, 10(-7)-10(-6) M), induced an increase in basal tension which remained after removal of endothelium or in Ca(2+)-free solution, but was inhibited by beta-flunaltrexamine. beta-Flunaltrexamine also inhibited the contractile response induced by DAMGO added during the relaxation of the contractions induced by noradrenaline. The delta-opioid receptor agonist, [D-Pen2,D-Pen5]enkephalin, had no effect on resting tension but potentiated the contractions induced by noradrenaline; these effects were abolished by naltrindol. The selective kappa-opioid receptor agonist, bremazocine, had no effect on resting tension and did not modify the amplitude of the contractions induced by noradrenaline. These results suggest that, at low concentrations, agonists of mu- and delta-opioid receptors may act as modulators of noradrenaline-induced responses, whereas at higher concentrations, mu-opioid receptor stimulation may have a direct contractile effect in isolated rat aorta.