This paper compares laboratory methods for the conditioning of concrete prior to exposure to the standard fire test (ASTM Method E 119) using slab specimens 3 by 3 ft by 6 in. thick. In the artificial conditioning methods the specimens were exposed to heated air in a kiln controlled at temperatures to 200 F. Such kiln drying was conducted both without controlled humidity and at several selected relative humidity (RH) levels. The drying of some slabs was accelerated by infrared heat radiation. Information is given on the effect of ambient RH level in the natural drying procedure described in Method E 119. The effect of the several drying procedures on the time requirements to reach the desired concrete test humidities, the humidity gradient through the concrete sections, and the subsequent fire endurance of the concrete slabs was evaluated in terms of the results obtained on companion slabs naturally dried in air at 73 F and 35 per cent RH. It was found that all of the artificial conditioning methods considerably reduced the time required to reach test humidity. However, the saving in conditioning time was accompanied by depressed fire endurance periods, that is, endurance periods substantially lower than those resulting from slabs naturally dried to the same test RH.