The invention relates to a control method and a control arrangement which comprises a feedforward power amplifier (10), which receives and amplifies a signal and which on amplifying the signal causes non-linearities to the signal. The control arrangement also comprises adjusting members (50, 80) and adaptation means (110, 130), which form an adaptation factor and control the operation of the adjusting means (50, 80) by means of the adaptation factor by adapting the amplitudes and phases of signal propagating along different signal paths to one another. The control arrangement further comprises control means (120), which produce information of the gain required by the power amplifier (10), said control means (120) ordering the adaptation means (110, 130) to adjust the adaptation factor in advance on the basis of the received information to correspond to the gain of the power amplifier (10), which enables adaptation of the signal also when the input power level changes rapidly.