When an intense acoustic stimulus is presented to the rat in a background of steady random noise, the startle reaction is larger than when the same stimulus is presented against a background of silence. Two experiments examined the temporal parameters of this facilitation effect. The initial experiment sought to determine how long the background signal must be present before facilitation can be detected. The second experiment examined the question of how long facilitation persists following termination of a background signal. Results indicated that 100 msec of exposure to a background signal is adequate for full development of the facilitation effect, and that the effect exhibits a significant decline by no more than 8 msec after the termination of a background signal.