Holding breath combined with immersion of face in cool water induces diving reflex consisting in bradycardia, peripheral vasoconstriction, and preferential redistribution of blood to the vital organs. The individual responses are variable, highly expressed in young children to weaken with age. Detailed assessment of the cardiac response to diving in late adolescents who are mostly prone to unexpected diving experiments and related complications are of practical importance. The study was performed to the investigate the heart rate (HR) response to controlled breath-hold diving in cold water in young healthy late adolescents aged 17.5 - 20.4. Twenty volunteers performed a procedure consisting in immersion of face in cold water during possibly long breath-holding. Beat-to-beat HR was assessed from the continuous ECG recording. Average diving time (tD) was 77.5±10.5 s, ranging from 61.7 to 95.4 s. Apnea and face immersion induced a biphasic response: an increase in HR was followed by its gradual decrease to reach a final limit of the response 47.7±9.0 beats/min at 55.2±16.1 s of dive. Within a variable time course of the evoked responses, two patterns of the HR decrease were distinguished: monoexponential decay functions fitted best for 8 (40%) subjects with a constant rate τ 31.3±11.4 s-1, whereas the 60% majority displayed a two phase negative chronotropic effect with constant rates: τ1 10.3±4.2 s-1 and τ1 25.5±10.1 s-1. The initial, anticipatory excitation influenced the HR response: the higher was the initial HR increase, the more pronounced was bradycardia. Disturbances of heart rhythm were observed in majority of subjects: supraventricular ectopic beats (40%), ventricular beats (20%), I-degree AV block (30%), II-degree Mobitz I block (15%), junctional rhythm (60%). In one case pair of ventricular ectopic beats looked severe and the trial was rapidly terminated. In conclusion, the unquestionable potency of late adolescents towards diving challenges, accompanied with a vigorous emotionally driven initial cardioexcitation meets a relevant vulnerability for arrhythmias.