A method and an apparatus for controlling the viscosity of a mixture by controlling the volumetric flow of a fluid with respect to the volumetric flow of another fluid is disclosed. The apparatus includes volumetric flow detectors for detecting the flow of the two fluids. The apparatus also includes an electronic controller device which responds to electrical signals generated by the volumetric flow detectors in proportion to the respective detected volumetric flows. The output of the controller device operates a flow drive element, such as a pump, so that the volumetric flow of one of the fluids is controlled. The volumetric flow of the controlled fluid and the volumetric flow of the other fluid are mixed by a suitable mixer device to obtain the mixture having the desired viscosity.