Subjects were required to discriminate previously learned "standard" versions of angular shapes from randomly perturbed "distractor" versions that varied in similarity to the standard. Advance information concerning the identity and the orientation of the test form was provided. Subjects were instructed to prepare for the presentation of the test form by mentally rotating an internal representation of the designated standard form (identity cue) into the designated orientation. The time needed to prepare for the presentation of the test form increased linearly with the angular departure of the indicated orientation from a previously learned position. This finding suggests that, in accordance with instructions, subjects performed a mental rotation in preparing for the upcoming test shape. Rate of preparation was not affected by the complexity of the standard form presented as the identity cue. DIscriminative departure of the test form presented as the identity cue. Discriminative reaction time was not affected by either test-form complexity or angular departure of the test form from the learned orientation. In addition, striking individual differences in the pattern of discriminative reaction times were found. The implications of these results for (a) the nature of the processes and representations underlying the mental rotation task and (b) the nature of visual comparison processes are discussed.