Brain death is a pathophysiological condition associated with major hemodynamic changes, temporary myocardial ischemia, and histological damage of the heart. These modifications could be related to a major local release of norepinephrine from myocardial sympathetic nerve endings leading to norepinephrine cardiotoxicity. This study was designed to evaluate the utility of cardiac microdialysis to measure interstitial myocardial norepinephrine release resulting from brain death. The dialysis probe consisted in a 10×0.20-mm dialysis fiber with a 18,000 mol wt cutoff. Dialysis probes were implanted into the right and left ventricular walls of the beating heart in anesthetized pigs and perfused with Ringer solution at 2 µl/min. Dialysate norepinephrine concentration was measured using HPLC with electrochemical detection. The relative recovery rate of norepinephrine in vivo was 34±4%. Interstitial fluid concentrations were obtained using the following formula: [C]interstitium = [C]dialysate/Recovery in vivo. After brain death, a transient increase in interstitial norepinephrine concentration was observed (from 0.74±0.20 to 4.50±0.60 ng/ml and 0.76±0.20 to 6.2±0.9 ng/ml in left and right ventricle, respectively, P<0.01) which far exceeded plasma level increase (from 0.50±0.10 ng/ml to 0.91±0.20 ng/ml, P<0.05). This increase in myocardial norepinephrine was, moreover, biphasic, with a second peak occurring 40 min after brain death. The present study confirms the onset of a dramatic increase in cardiac norepinephrine release from myocardial nerve endings following brain death, and demonstrate the utility of the new cardiac microdialysis technique to assess changes in interstitial fluid content.