Abstract: Apparatus for treating organisms for therapeutic purposes consists in applying thereto by means of electrodes an electrical signal comprising successive pairs of pulses, the pulses of each pair being of opposite polarity. The apparatus has an electrode arrangement for application to the organism and a signal generator for supplying to the electrode arrangement the signal comprising successive pairs of pulses, the pulses of each pair being of opposite polarity. The generator has controls for varying independently of one another the amplitude of the pulses of each polarity, the duration of the pulses of each polarity, the interval between the pulses of each pair, and the interval between successive pairs of pulses.