Coincidental stimulation of the parasympathetic nervous system (face immersion and breath holding) and sympathetic nervous system with (anxiety and cooling) can cause “autonomic conflict” and consequent ECG abnormalities that are usually supraventricular and asymptomatic (Tipton et al, 1994). In theory, autonomic conflict could occur during HUET; the present study tested this possibility. Given the large number of people undergoing HUET training each year, and the absence of any reported cardiac problems, it was hypothesised that either ECG irregularities do not occur or, if they do, they are asymptomatic Following ethical approval and informed consent, 26 naïve males completed a HUET submersion into water at 29.5°C. Each submersion was standardized: the participant entered the HUET helicopter and was secured into the seat with a four point harness. This plus the ditching briefing took 90s. At 3.5 minutes the dunker was submerged and rolled to the inverted position, this took 10s. Once inverted the participant escaped, this took an average of 10s during which they breath held. They then floated supine until 4.5 minutes had elapsed. Participants wore a three lead (V5) telemetric ECG system (Sharktooth, MIE). They wore underclothing and immersion dry suit. Skin temperature was measured in one subject on the chest, forearm, scapula and forehead. The ECG trace was examined independently by a clinician and thermal physiologist. Participants had raised heart rates prior to being submerged, indicating sympathetic activation. Heart rate increased during the HUET more probably due to anxiety and physical effort than cold shock as skin temperature did not fall. Participants demonstrated a range of cardiac arrhythmias the most prevalent being: bradycardia; premature junctional escape; and ventricular ectopics. 26 of these arrhythmias were observed in 19 different participants; 23 of the arrhythmias occurred just after the cessation of breath holding. Concurrent stimulation of the sympathetic and parasympathetic nervous system during HUET results in ECG abnormalities. The timings of these (after the release of a breath hold) is consistent with earlier findings (Tipton et al, 1994). These ECG abnormalities are asymptomatic and probably of little clinical significance in young, fit individuals. It remains to be seen if this is the case with an older, less fit cohort of people.