Abstract: Method for the controlled injection of a liquid in a tube by means of a peristaltic type pump actuated by a rotary d.c. motor (12) controlled by a control unit (10) such as a microprocessor, wherein an optical encoder (18) sends pulses to said control unit (10) during the intermittent operation of the motor. The method comprises operating the motor for period corresponding to a number of pulses equal to a set value. For each operating phase of the motor (12), equal to a predetermined number of pulses, the set value is reduced by the number of pulses transmitted during the preceding deceleration phase when the power supply is interrupted. The resulting error in the level of the volume of liquid to be injected thus remains approximately constant for the entire length of the injection period. The method according to the invention may be advantageously used in a programmable portable infusion system.