Abstract: An apparatus is provided for permitting commercial-off-the-shelf (COTS) electronics to be utilized in harsh environments for which the COTS electronics are not designed, including environments having ambient temperature variations, ambient pressure variations, shock and vibration exposure, environmental contaminants and/or electromagnetic radiation which may be beyond the design specification of the COTS components. The device includes a sealed enclosure having racks or other suitable mounts for the COTS components and having an environmental controlled unit (ECU) which circulates a fluid such as air through the enclosure and through the mounts. The temperature in the enclosure is monitored and the ECU operated to heat or cool fluid passing therethrough to maintain temperature in the enclosure within the design limits of the COTS electronics. A flow channel for ambient fluid, which channel is sealed from the internally circulating fluid, may be provided for removing heat from a cooling unit of the ECU.