After heart rate variability (HRV) had been established in the clinic, the question about the minimum interval length for analyzing electrocardiograms emerged. Respiration rate, heart rate and heart rate variability were analyzed in 25 sport divers during 6 min intervals at control, immersion, submersion and while SCUBA diving. Thereafter, the interval length was systematically shortened to 1 min. RESULTS: Respiration rate was significantly reduced during submersion and diving. Heart rate, in turn, remained essentially unchanged during the four experimental steps. The HRV measures in the time domain (standard deviation, coefficient of variation, RMSSD and pNN50) exhibited significant changes during immersion, submersion and diving compared to control conditions. The spectral density in the low frequency range was increased compared to control, the increase being significant during diving. Immersion, submersion and diving, thus, present strong stimuli for the autonomic nervous system. The length of the HRV measures of the time domain could be shortened to 3 min without significant loss of information, except for pNN50. Reduction of the respiration rate during diving considerably shifted the respiratory arrhythmia from the high to the low frequency range. Such shifts deserve special attention interpreting HRV measures from the frequency domain. The interval length for the measures in the frequency domain could only be shortened to 5 min. CONCLUSION: Measures from the time domain, in particular standard deviation and co-efficient of variation, seem to be superior to measures from the frequency domain in analyzing short-term HRV.