Step-by-step controllable thermal reduction of individual graphene oxide sheets, incorporated into multiterminal field effect devices, was carried out at low temperatures (125-240 degrees C) with simultaneous electrical measurements. Symmetric hysteresis-free ambipolar (electron- and hole-type) gate dependences were observed as soon as the first measurable resistance was reached. The conductivity of each of the fabricated devices depended on the level of reduction (was increased more than 10(6) times as reduction progressed), strength of the external electrical field, density of the transport current, and temperature.