Background: ST segment deviation is common in patients with narrow QRS complex tachycardia. It mainly concerns young people in whom ischemic background is unlikely. In our work we would also like to propose another potential mechanism – the overlapping of individual components of the QRS complex. Material and methods: The study included 11 patients (7 women and 4 men) with paroxysmal narrow QRS complex tachycardia. In all patients EPS was performed and the diagnosis of atrioventricular reentrant tachycardia was established and finally successful RF ablation was done We mesured the individual components of QRS QR, RS and RJ during sinus rhythm and during tachycardia. Results: The difference RJ-QR during tachycardia correlated negatively with tachycardia cycle length T(r = -0.85, p = 0.000831 ). We also showed a significant difference between the amplitude of the RJ segment in tachycardia and during sinus rhythm (p = 0.005), at the same time we showed no differences between the amplitude of QR and RS. Conclusions: We showed a statistically significant difference in ST segment depression in correlation with the rate of tachycardia in patients with AVRT resulting mainly from the overlapping of individual components of the QRS complex.