A mercury vapor discharge lamp such as a negative glow discharge lamp includes a light-transmitting envelope containing a noble gas fill material. An anode electrode and a cathode electrode are spacedly located within the envelope. An amalgam-forming material for regulating the mercury vapor pressure of the lamp is disposed within the envelope. In order to increase the mercury vapor pressure and consequently the light output of the lamp at low ambient temperatures, a heating element is disposed within the envelope and in close thermal proximity to the amalgam-forming material. A thermostatic switch coupled to the heating element is activated when the temperature adjacent the amalgam-forming material is below a predetermined temperature.