Circuit for automatic dynamic compression an/or expansion. The circuit has adjusting member (10) for varying the propagation constant in a useful signal path and a further similar adjusting member (30) for varying the propagation constant in a branch path which serves for deriving a control voltage which is applied to the control inputs (11,31) of the adjusting members (10,30) in the useful signal path and in the branch path. Further the circuit has a lowpass circuit (20 . . . 26) which is connected to the adjusting member (10) located in the useful signal path. The circuit can be changed over to a different compander characteristic when the adjusting member (10) located in the useful signal path is connected up as a controllable highpass (10,15). The limiting frequency is of the order of magnitude of the limiting frequency of the lowpass circuit (20 . . . 26). The highpass can be formed by a capacitor (15) in series with the adjusting member (10) in the useful signal path and the highpass function for low frequencies can be limited by a resistor (12) in parallel with the adjusting member (10) and the capacitor (15). The limiting frequency of the lowpass circuit (20 . . . 26) is reduced by an additional capacitor (27) in parallel with the capacitor (26) already provided. In both operation modes, the same voltage generator (30 . . . 36) is used for controlling the adjusting member (10) in the useful signal path.