Three experiments investigated the effect of stimulus probability on same-different classification time. In Exps I and II, Ss made same responses on the basis of name matches of simultaneously presented letters. Half of the same trials involved letters that were also physically identical. Exp I shows that the presentation probability of specific letters affected name matches and different responses, but not physical matches. Exp II varied stimulus contrast as well as probability. Contrast had a main effect but did not interact with probability at any level of processing. In Exp III, Ss were switched to the physical level of processing. Stimuli that had the same name but differed in case were called different. In this condition, the probability effects observed in Exp II disappeared. These results are interpreted as demonstrating that stimulus probability has its effects during the process that derives the name of the stimulus from the visual representation. This process takes place before the name comparison is made, and the name comparison process precedes the determination of the different response.