A study of the atrioventricular (AV) conducting tissue was considered necessary for the examination of probable histologic changes that could justify the arrhythmias observed in street-heroin addicts. Postmortem coronary angiography and microscopic examination were performed in 50 heroin addicts (group A) and in 50 nonaddicts (group B), all male 16–40 years old. In group A, fatty and/or fibrous tissue replaced the AV node in 50% of cases while in group B in 14%. The main bundle was replaced by fatty and/or fibrous tissue in 44% in group A cases and 10% in group B. Intimal proliferation and fibromuscular dysplasia of the AV arteries in group A were correspondingly 26% and 14% and in group B 6% and 2%. Inflammation with focal and/or diffuse concentration of round cells of the AV node was detected in 54% in group A. These findings could explain a possible arrhythmia mechanism in this population.