The experiment aimed to test in a spontaneous object recognition (SOR) task the effects of stimulus similarity and the interval between a previous familiarization trial and such a task. 1h or 24 h after the familiarization trial with two identical copies of an object, exploration of one of the objects and another novel object was assessed by recording the number of approaches, as well the time spent close to them. On the test the stimuli could either differ slightly in form (Difficult discrimination condition) or markedly in form and color (Easy condition). Subjects in the Easy but not the Difficult condition preferred to explore the novel object regardless of the retention interval (1h or 24h). After the 24h interval, exploration of the novel stimulus was greater for subjects in the Easy than the Difficult condition but they did not differ in terms of exploring the familiar object. Thus, rats in the Difficult condition might remember the familiar stimuli as well as rats in the Easy condition after 24 h, but were unable to distinguish it from the novel stimulus. The implications of this notion for using the SOR task as a procedure for studying perceptual learning and memory are discussed.