Application of a steady electric field to single crystals of ZnS: Mn, Cl excited by 10 kev electrons produces enhanced yellow emission and reduced blue or green emission. Increases of up to five times in yellow luminescence have been observed on application of 100 v. Localization of the effect at the cathode has been observed visually. Observations under pulsed conditions have shown that application of a field raises yellow emission to a steady level of enhancement. Quenching of green emission reaches a maximum immediately after application of the field and then decreases to a continuous lower level; a small enhancement pulse appears on removal of the field. Crystals of ZnS: Cu, Cl, and those without intentionally added activator, have shown only quenching of luminescence by the application of a field; this occurs both in those crystals which show electroluminescence due to alternating fields and in those which do not. Electroluminescence appeared in some manganese activated samples under direct voltages as small points of yellow light at the cathode; however, in copper activated crystals points and streaks of blue light appeared under alternating voltages at precipitates throughout the crystal bulk. Possible mechanisms for the enhancement and quenching of cathodoluminescence are discussed.