The aim of the present research was to analyze the acute psycho-physiological response during a high intensity interval training (HIIT) session of trained swimmers. We analyzed blood lactate concentration, heart rate, heart rate variability (HRV), arms isometric strength, rating of perceived exertion (RPE) and cortical arousal before and after a HIIT session in 14 trained swimmers (16.2 +/- 2.6 years 169.1 +/- 10.2 cm 61.3 +/- 9.9 kg). HIIT session consisted in: 4 x 10 m tethered swimming resting 90 s between sets, 3 min rest, 16 x 25 m maximum speed swimming resting 30 s between sets. Blood lactate concentration, cortical arousal, and rating of perceived exertion significantly increased (p < 0.05) after HIIT. HRV parameters significantly decreased after HIIT, showing an increase in sympathetic nervous system modulation. Results obtained showed the high impact of HIIT sessions on the swimmer\'s organism, which may be the cause of adaptation in this low volume training sessions.