Abstract: The ink jet printing system contains at least one ink jet printing head operatively connected to printing medium reservoirs by a conduit system. A partial ink flow is passed through a regulating apparatus for viscosity-dependent regulation. On exceeding the ink viscosity or ink density, a spindle body constructed as a float contacts a switch constructed as a proximity switch. The switch is operated such that a corresponding signal is supplied to a control valve and as a result the supply of solvent to a main ink reservoir of the printing medium reservoirs is initiated. The regulating apparatus may constitute an independent assembly associated with the main ink reservoir or can be integrated into the main ink reservoir.