The habituation of locomotor activity across repeated exposures to a novel maze was studied in a series of experiments using rats as subjects. Habituation, defined as a decrease in ambulation, was greater on a second trial occurring 5 min after a first trial than on one occurring 60 min after. This short-term decrement occurred only when the same maze was used on both trials, and could be dishabituated by intertrial detention in another novel environment. On a delayed test trial, habituation was, in one case, somewhat greater following initial spaced trials, and in another condition, comparable following both massed and spaced trials. The longer term habituation was maze specific, but was not affected by the presence of a dishabituator following either or both of the first two trials. The results were discussed in terms of theories of "priming" and encoding variability.Much of the recent interest in the response decrement that occurs with repeated exposures to a stimulus (i.e., habituation) has focused on the short-term effects of prior stimulation. For example, Davis (1970) found more habituation of the startle response of rats during a sequence of massed tone presentations than during spaced presentations. Whitlow (1975), using a vasomotor component of the orienting response in rabbits, observed a short-term response decrement due to an immediately prior stimulus occurrence, that was separable from the longterm habituation due to the cumulative history of stimulations. Whitlow also found that the short-term decrement occurred only with repetitions of the same stimulus, and that it could be removed by placement of a dishabituating stimulus between repetitions of the target stimulus.However, as Davis (1970) and Whitlow (1975) also noted, the long-term effects of such manipulations can differ from the short-term effects. For example, Davis found, on delayed test trials, that initial spacing of the stimulus presentations led to more habituation than did the initial massed presentations.One theory of both the short-and long-term response decrements of habituation is Wagner's (1976) theory of priming in short-term memory (STM). According to this theory, a stimulus would be less effective in eliciting an unconditioned response if it occurred while a representation of that stimulus was