The degree of response suppression was measured in 5-sec intervals during a signal that was associated with electric shock. Four groups of 6 naive male albino Norway Charles River CD rats were tested with signals of fixed or random duration and with shocks that occurred at the end of the signal or at random times during the signal. In all cases, there was a transient effect of the onset of the signal, but thereafter, the 4 groups showed qualitatively different fear gradients. The magnitude of fear was closely related to the expected time to the next shock. (PsycINFO Database Record (c) 2006 APA, all rights reserved)