Third degree atrioventricular (AV) block in dogs is thought to be most frequently characterized by non-specific fibrotic changes in the AV node. However, it may occur secondary to an undiagnosed inflammatory process. We report a case of third degree AV block in a dog, secondary to acute lymphocytic-plasmacytic myocarditis that resulted in sudden death. This dog had cardiac troponin I levels of 44.65 ng/mL (normal <0.11 ng/mL). The serum cardiac troponin I level was five times higher than any other AV block patient measured in our laboratory, and was also substantially higher than in dogs with chronic valve disease or dilated cardiomyopathy. The severe myocardial necrosis observed at necropsy correlated with the degree of cardiac troponin I elevation. This report suggests that measurement of cardiac troponin I may be an indicated test before pacemaker implantation in dogs with third degree AV block.