Case report Junctionnal ectopic tachycardia (JET) is a rare cause of supraventricular tachycardia. We report two unusual cases : one transient in a neonate and one in an adolescent. § Case1 presented JET after birth with normal echocardiography. After a few hours, tachycardia resolved without any treatment. § Case2, an asymptomatic adolescent, presented arrhythmia on routine physical examination with JET on ECG, normal echocardiography and constant JET on Holter. During exercise, sinus rhythm dominated and JET reappeared during recovery phase. JET is a rare arrhythmia caused by an ectopic focus usually perinatal or following heart surgery, radiofrequency ablation of AVNRT and digoxin toxicity. In neonates, it’s a severe disease often refractory to medical treatment, except amiodarone and flecaïnide, with focus disappearance after months or years but only after a few hours in our case. In case2, JET appeared unusually at 13 years. No treatment was given since heart rates were not too high but average heart rates and cardiac function will regularly be controlled. Conclusions: we describe unusual forms of JET: one transient in a neonate and one in an asymptomatic adolescent.