The paper gives, in concise form, the most important rules for the rational determination of bit number and sampling rate in the analogue-digital (and vice versa) conversion of physiological signals. Guidelines for estimating the r.m.s. value of the noise and of the signal bandwidth are given. It is shown that, for digital-analogue reconversion, the sampling rate has to be chosen sufficiently above the Nyquist frequency. Finally, a procedure for estimating the bandwidth of a quasiperiodic signal is given.