Water spray is one method of protecting glazing exposed to fire. Research conducted at the National Research Council's National Fire Laboratory indicates that, under ideal conditions, a window assembly, when protected by special sprinklers, will stay intact, and can obtain a fire resistance rating of 2 hours or longer. Experiments showed that there are several parameters related to sprinkler operation which affect the performance of the glazing assembly under fire exposure. This paper describes the test facility and discusses the results of investigations conducted to determine the effect of the sprinkler activation time and sprinkler water flow rate on the effectiveness of dedicated sprinkler systems for protecting large glazing assemblies. The effect of the sprinklers on the average temperature and temperature gradient in the test room is also discussed.