A hydraulic fluid control system for controlling pressure fluid supplied to consumers (2, 3, 4) of a rock drilling machine, a regulating valve (6, 7, 8) for each for each consumer and fluid conduits between the regulating valves and the consumers, an electronically controlled auxiliary control unit (11) having an electrically controlled auxiliary valve (14) for connection to a fluid conduit, a sensor for sensing fluid parameter values and sending sensor signals to the auxiliary control unit as sensor input signal actual values, micro-controller (12) having at least one parameter sensor input signal entry (S1-S5, I1-I5) for receiving the sensor input signal actual values and a control signal exit (V1-V6) for signal control of an auxiliary valve, the processor (12) is being arranged to compare the actual and predetermined values and to emit control signals to an auxiliary valve in response to the comparison to adjust fluid flow.