Objectives: Recently, extremely high levels of endothelin-1 (ET-1) were detected in the pericardial fluid of patients with heart disease; however, the pathophysiological importance of this finding is not known. The present study was designed to characterize ET-1 levels in canine pericardial fluid and to investigate the effects of local high concentrations of exogenous ET-1 in vivo. Methods: In anesthetized, open-chest dogs ET-1 (Groups 1 and 2: 11 and 33 pmol·kg⁻¹·min⁻¹; n=6 and 6, respectively) or physiological saline (Group 3, n=5) were infused into the closed pericardial sac for 40 min. In serial pericardial fluid and aortic blood plasma samples, ET-1 levels were measured by radioimmunoassay, and analysed by high-performance liquid chromatography (HPLC). Systemic arterial blood pressure, heart rate, cardiac output (CO), standard ECG and right ventricular endocardial monophasic action potentials (MAPs) were recorded. Results: Basal pericardial fluid ET-1 levels were significantly higher than respective plasma levels (342±210 vs. 8.0±5.2 pmol·l⁻¹, n=14, P<0.001). In HPLC analysis pericardial fluid ET-1 was indistinguishable from ET-1_1–21. Infusion of exogenous ET-1 into the pericardial space induced ventricular arrhythmias in all instances, which were associated with 9.7-fold increase in pericardial fluid ET-1 levels. Ventricular tachycardias developed in 9 of 12 animals. The arrhythmogenic effect of ET-1 was more apparent in dogs with the larger dose. Before the onset of arrhythmias, intrapericardial infusion of ET-1 increased QT time (Group 1: 207±18 to 230±23 ms, P<0.01; Group 2: 220±12 to 277±17 ms, P<0.01) and MAP duration at 90% repolarization (at 300 ms cycle length) (Group 1: 192±9 to 216±9 ms, P<0.01; Group 2: 205±9 to 255±9 ms, P<0.001). Hemodynamic variables did not change significantly prior to the onset of ventricular tachyarrhythmias. In Group 3, arrhythmias were not observed and all electrophysiological and hemodynamic parameters remained unchanged. Conclusions: Administration of exogenous ET-1 into the pericardial space induces ventricular arrhythmias associated with prolongation of QT time and MAP duration. Whether pericardial fluid ET-1 under pathophysiological conditions can ever reach sufficiently high levels to induce ventricular arrhythmias remains to be elucidated.