The effects of fentanyl on sympathetic nerve endings in canine coronary arteries have been examined. Measurements were made of change in endogenous norepinephrine in isolated coronary artery in the presence of fentanyl, and of the effects of fentanyl on norepinephrine release and metabolism using superfused strips of coronary arteries in which transmitter stores had been prelabeled with [3H]norepinephrine. [3H]Norepinephrine and its 3H metabolites were separated from samples of superfusate (physiologic salt solution flowing continuously over the prelabeled coronary artery) by column chromatography and measured by scintillation spectrometry. The addition of fentanyl (2 x 10(-6) and 8 x 10(-6) M) caused the overflow of total radioactivity (radiolabeled norepinephrine and radiolabeled metabolites of norepinephrine entering the superfusate) to increase significantly above basal levels in both unstimulated and electrically stimulated arteries. The increased overflow could be accounted for by a small increase in norepinephrine and a sizeable increase in 3,4-dihydroxyphenylglycol, the metabolite arising from the intraneuronal metabolism of norepinephrine. Because this metabolite is without agonist activity and the increases in norepinephrine were small, fentanyl in the concentrations studied had minimal effects on adrenergic neurotransmission in this vascular bed.