Since only electrons which have relatively high energies can escape from a metal, a thermonic emission current, when initiated, reduces the average energy of the remaining electrons and molecules and hence, if not compensated for, the temperature of the emitter. Investigations of several diagnostic x-ray tubes and generators indicate that there is a noticeable, exponential fall-off of x-ray tube current during an exposure (time constant = 130 msec) attributable to this phenomenon. With three-phase generators the equilibrium values observed were approximately 15% lower than the corresponding values at the beginning of the exposure. Smaller fall-offs were observed with single-phase equipment. Theoretical expressions derived from basic thermodynamic considerations, which are in good agreement with observations for the magnitude of the effect and its time constant, are presented. © 1977 SPIE.