A vehicular automatic transmission control system comprising: a hydraulic control circuit including a hydraulic pressure source, a pressure regulator valve for producing a line pressure by regulating the oil pressure supplied from the pressure source, a number of shift valves for selectively engaging and releasing friction members of the automatic transmission according to operating conditions of a vehicle such as vehicle speed and throttle opening rate, and a solenoid valve blocking switching operation of a highest shift gear toward a highest gear position of the transmission; and an electronic control circuit for controlling on- and off-ranges of the solenoid valve, including a manual mode switch for setting the control either in a highest speed non-cancel mode permitting shifts to the highest gear position or a highest speed cancel mode inhibiting shifts to the highest gear position and forcibly effecting a downshift when selected during operation in the highest gear position, and a vehicle speed sensor; characterized in that: the electronic control circuit is arranged to nullify setting of the highest gear cancel mode by the manual mode switch at a vehicle speed higher than a predetermined value.