A method is disclosed for preparing and dispensing a mixture obtained by mixing at least one first chemically reactive component and at least one second chemically reactive component containing a dispersed solid material i.e. a filler, by a high pressure mixing device comprising a head provided with injectors and one or more interception and supply devices defined by respective nozzles for narrowing the passage of the flows and with correlated plug members that are movable axially and settable for controlling said narrowing. The injectors and the interception and supply device transform the pressure energy of the fluids into kinematic energy of the jets that clash in the mixing chamber to mix the components for turbulence induced by the jets, in which a valve member is slidable, in particular a slide-valve, provided with longitudinal slots for recirculating the components to respective storage tanks. The method provides: removing from at least one tank, by pumping arrangement connected to a respective hydraulic delivery line to the head, a dosed quantity of the at least second chemically reactive second component to with filler material is added; preventing, once said quantity is removed, that the second component flows back from the pumping arrangement to the tank; dosing the component with a controlled flowrate B to the delivery line to the mixing head; keeping the slide-valve closed in a recirculating position and preventing the entry of the at least second component to the head and to the respective recirculating slot and pressurizing, by a dosed quantity Vot, the delivery line to subject the at least second polymeric component to a pressure increase along the delivery line until a value is reached that is near that reached by the passage through the nozzle of the respective interception and supply device; subsequently enabling the entry of the second component into the head to make the second component recirculate to the slots of the slide-valve for a period of time that is comparatively very reduced until it is reduced virtually to zero with respect to a recirculating step of the at least first component through the slide-valve, and subsequently opening by retracting the slide-valve to permit mixing by high pressure collision of the first and at least one second components to dispense the resulting mixture through the mixing chamber and the dispensing conduit for pouring the mixture into a cavity or into a shape to make an object by reaction of the aforesaid first and second chemically reactive components. An apparatus for preparing and dispensing the mixture is also disclosed.