A anti-lock brake control system and/or a drive slip control system is described the slippage value of which for the brake pressure control is to be set close to the maximum of the .mu.-slippage curve, on the stable branch, that is, during the control.For this purpose, a short pressure pulse is generated in a phase where the pressure is maintained constant and the difference of the reference speed and the wheel speed is integrated (J) over a time T.sub.I and the maximum of the difference (D.sub.max) is determined in this time. The two values are divided ##EQU1## and the resulting values is used for determining the slippage.