IN TIME domain analysis, and especially in haemodynamical studies, it is usual to average the measured parameters over a number of cardiac cycles in order to reduce (a) measurement errors and (b) the influence of the normal variety in heart beats. Obviously, in frequency domain analysis the results should be averaged too. It was the purpose of this study to compare the frequency spectra obtained by applying different averaging methods either in the time domain or in the frequency domain. In addition, the number of harmonic components which still contribute significantly during reconstruction of the analysed signal was determined. The necessity of averaging is obvious as the frequency of the harmonic components is directly related to period length and hence different spectra may be obtained due to the normal variability in heart rate. ATTINGER et al. (1966) have pointed out the differences in frequency spectra obtained by faulty indication of period length. Moreover, the greater variability in moduli of the venous pressure pulses with respect to arterial pressure has been shown by PATEL et al. (1965). This note deals with the frequency spectra of venous pressures observed during different conditions of increased filling pressures, conditions which are frequently present in patients with congestive heart failure.