Ventricular resection of the heart of Ambystoma mexicanum was performed and the type of tissue that restored the lesion and if it is by hypertrophy or hyperplasia of myocardium, were evaluated. Masson's trichrome stain indicated that 5 days after resection, the gap was occupied with a blood clot surrounded by collagen fibres (83 +/- 6%) and muscle (10 +/- 3%) and the rest of area (7 +/- 2%) free of tissue. A proportion of 50 +/- 4 and 90 +/- 2% was muscular tissue, 10 and 30 days after injury. The evaluation with bis-Benzimide indicated cell proliferation in the injured area. The double immunohistochemistry for alpha-sarcomeric actin and proliferating cell nuclear antigen indicated that the tissue that occupied the injury-produced gap was originated by cardiomyocyte proliferation, which presented a maximum of 68%, 5 day after injury. Our results indicate that the myocardium of A. mexicanum recovers its structure through cardiomyocyte hyperplasia and suggest that the myocardial regenerative capacity is higher than the reported for adult mammals (1%) and other non-mammalian vertebrates (32%). This characteristic makes A. mexicanum a suitable model to study the mechanisms that regulate per se, the myocardial regeneration in adult vertebrates in vivo.