A system for controlling the hydraulic pressure of a hydraulic control circuit of an automatic, variable speed transmission during a shift operation is provided. The hydraulic control circuit includes a plurality of hydraulic pressure control valves, a plurality of flow path switching means and a plurality of solenoid valves for controlling the engagement and disengagement of clutches and brakes in the transmission. The energization of the solenoid valves disposed in the hydraulic control circuit is controlled by an electronic system, which comprises a large scale integrated semiconductor logic unit, a semiconductor read-only memory, a semiconductor read-write memory and as required, input/output ports. The semiconductor read-only memory stores, inter alia, hydraulic pressure control program data which become operative during a selected one of the shift modes, each of which comprises a combination of the current speed stage and the speed stage to be established, and which controls the hydraulic pressure of the hydraulic control circuit by a repeated deenergization and energization of the solenoid valve for a given time interval during the shift operation.A signal indicative of a shift lever position is applied to the electronic system from a shift lever detecting means. Also a signal indicative of the current vehicle speed is applied thereto from a vehicle speed signal generating means. Finally, a signal indicative of a throttle opening is applied to the system from a throttle opening detecting means. In response to these signals and based on the constants data and the program data which are stored in the read-only memory, the electronic system determines a speed stage to be established. The hydraulic pressure in the hydraulic control circuit is controlled, simultaneously with a switching of the speed stage, on the basis of the hydraulic pressure controlling program data, thus reducing any rapid fluctuation of the vehicle speed or impulses during a shift operation.