The characteristics of some EMI photomultipliers when operated continuously with large cathode and collector currents are described. It is shown that cathode currents up to about 10 &mgr;A/cm2are possible with some types of tubes, the only limitation being cathode deterioration under intense illumination. In other types surface resistivity and space charge prevent operation above about 0.02 &mgr;A/cm2. Data on collector saturation are given and the effect of dynode heating by large currents with consequent reduction in secondary emission factor is also described.