Abstract: In tuning a regulator (2) of the PID-type of a process (1) in a feed back system where the process and the regulator has a transfer function G(s) in common, a method is proposed for bringing the system into self oscillation for measuring the amplitude and frequency of the oscillation and tuning the regulator in dependence of the measurements obtained. A circuit function (NL) having non-linear characteristic and a describing function N(A) is introduced into the system in series to the process for acting on the regulator signal (e). Self oscillation is obtained if G(i.omega.).multidot.N(A)=-1 for at least one value of the angular frequency .omega. and the amplitude A of the regulator signal (e). An apparatus for performing the method is disclosed.