Shipboard electrical installations present special problems for insulating materials that may not exist in shore installations. As naval vessels necessarily are built to a high degree of compartmentation requiring forced ventilation and also have large capacity power plants installed in relatively small spaces, there is perhaps more insulation per cubic foot of available engine room space than exists in land installations. Many naval vessels are complete cities in themselves. An electric fire in a surface ship is extremely dangerous — an electric fire in a submarine is a hazard. For this reason, all electrical-insulating materials used in naval vessels are screened at present carefully to determine their suitability at elevated temperatures and the effects of noxious gases produced on thermal decomposition of these materials. In order to determine the effects of temperature and operation time upon the production of noxious gases from silicone-insulated rotating equipment under condition of simulated submarine service, the Naval Medical Research Institute set out to find the answer to this problem for the Bureau of Ships.