The response of an RCA-4522 photomultiplier tube (35ET bialkali photocathode, 14 dynode stages) in the saturation region has been studied as a function of ambient background light intensity, photocathode illumination area, magnetic field orientation, and ambient temperature. These studies were made using various high-speed pulsed light sources. It is found that the phototube's response does vary with some of these parameters (in particular, photocathode temperature effects). However, its response is reproducible and highly successful operation in this region is possible through a series of calibrations.