A tire pressure alarm system which is mounted directly onto an existing lug bolt of a vehicle wheel after the bolt has been drilled axially to slidably accommodate a sensing rod. The sensing rod is restrained by a pressure responsive member, preferably in the form of a toroidal tube in direct fluid communication with the interior of a tire carried on the wheel. If tire pressure is lost, a spring overcomes the pressure responsive member to move the sensing rod to a position wherein it activates a signal circuit.