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Purpose The Janus Kinase 2 / Signal Transducer and Activator of Transcription 3 (JAK2/STAT3) pathway is involved in protection of the adult heart against ischemia/reperfusion. STAT3 activation can result from phosphorylation on two sites: tyrosine 705 by JAKs and serine 727 mainly by Mitogen-Activated Protein Kinases (MAPKs). We have previously shown that, under basal conditions, STAT3 is inhomogeneously distributed and phosphorylated in atria, ventricle and outflow tract of the embryonic heart. However, to what extent the JAK2/STAT3 pathway contributes to the response of the developing heart to transient oxygen lack or oxidant stress remains unknown. Methods Hearts isolated from 4-day-old chick embryos were studied in vitro. In a first series of experiments, hearts were submitted to anoxia (30min) and reoxygenation (80min) with or without the antioxidant MPG (1mM). The time course of STAT3 phosphorylation on tyrosine (P-Tyr) and on serine (P-Ser) was determined by immunoblotting on ventricles removed at specific time points. In a second series of experiments, hearts were exposed to H2O2 (1mM, 1 h) and STAT3 phosphorylation (P-Tyr and P-Ser) was assessed in atria, ventricle and outflow tract. Results In the ventricle, P-Tyr STAT3 was unchanged during anoxia but increased upon reoxygenation, reaching a peak (eightfold) after 1 h and returning to basal level after 80min. P-Ser STAT3 slightly increased (x1.7) only after 1 h of reoxygenation. The peak of P-Tyr STAT3 was abolished by MPG. Exposure to H2O2 increased significantly P-Tyr STAT3 only in the ventricle (twofold) but had no effect on P-Ser STAT3 whatever the investigated region. Moreover, H2O2 suppressed atrial activity in 50 % of the hearts and atrioventricular conduction in 70 % of the hearts. Conclusions In the anoxic-reoxygenated ventricle of the embryonic heart STAT3 activation i) is ROS-dependent, ii) requires specific phosphorylation on tyrosine site and iii) is probably independent of MAPKs. STAT3 could also contribute to the protection of the developing heart submitted to oxidant stress or transient anoxia.