The developing cardiovascular system is known to operate normally in a hypoxic environment. However, the functional and ultrastructural recovery of embryonic/fetal hearts subjected to anoxia lasting as long as hypoxia/ischemia performed in adult animal models remains to be investigated. Isolated spontaneously beating hearts from stage 14, 20, 24, and 27HH chick embryos were subjected in vitro to 30 or 60 min anoxia followed by 60 min reoxygenation. Morphological alterations and apoptosis were assessed histologically and by transmission electron microscopy. Anoxia provoked an initial tachycardia followed by bradycardia leading to complete cardiac arrest, except for the youngest heart which kept beating. Complete atrioventricular block appeared after 9.4±1.1, 1.7±0.2, and 1.6±0.3 min at stage 20, 24, and 27HH, respectively. At reoxygenation, sinoatrial activity resumed first in the form of irregular bursts and one-to-one atrioventricular conduction resumed after 8, 17 and 35 min at stage 20, 24, and 27HH, respectively. Ventricular shortening recovered within 30 min except at stage 27HH. After 60 min anoxia, stage 27HH hearts did not retrieve their baseline activity. Whatever the stage and anoxia duration, nuclear and mitochondrial swelling observed at the end of anoxia were reversible with no apoptosis. Thus, the embryonic heart is able to fully recover from anoxia/reoxygenation although its anoxic tolerance declines with age. Changes in cellular homeostatic mechanisms rather than in energy metabolism may account for these developmental variations.