A thermocouple was installed in the crown of a sodium-cooled exhaust valve and tested in an air-cooled engine cylinder to determine valve temperatures under various engine operating conditions. The highest temperature of the crown of the exhaust valve was 1337 deg F at fuelair ratio 0.064, indicated mean effective pressure, 179 lbsq in., engine speed, 2000 rpm and spark advance, 19 deg BTC. Variation in fuelair ratio had a relatively great effect on valve temperature, but cooling-air pressure and variation in spark advance had little effect. An increase in engine power by change of speed or mean effective pressure increased the valve temperature. It was found that the temperature of the rear-spark-plug busing was not a satisfactory indication of the temperature of the exhaust valve.