The time dependence of the gain of several RCA, DuMont and EMI photomultiplier tubes has been measured prior to, during, and after a constant illumination of the photocathode. The light source was a NaI(T1) crystal irradiated with γ-rays of Cs 137 or Co 60. A change in the counting rate results in rapid and slow variations in the gain. The rapid gain variations take place within an interval of less than 10 seconds. The amplitude is dependent upon the change in counting rate and the tube chosen. Moreover the sign may be positive or negative for individual tubes of the same type. It may be assumed that a sudden reversible sensitization or desensitization of the photocathode explains the effect. The slow gain variations after a change in counting rate are made up of two exponential components. The respective saturation value after an increase in counting rate is approached faster than after a decrease in counting rate. The amplitude is dependent upon the average anode current and the high voltage. The slow gain variations of all examined RCA tubes (6342, 6342A, 5819) were found to be negative, whereas the changes of the DuMont 6292 and EMI 6255 tubes were found to be positive. It may be assumed that reversible dynode effects explain the changes. The different behaviour of the various types of photomultiplier tubes is not correlated to the dynode material. The absolute value of the gain variations of all tubes examined were approximately of the same magnitude under comparable conditions.