Repetition effects were studied in a post-cue naming task, in which participants were cued to name one of two stimuli following their presentation. When pairs of pictures were repeated in a second block, former distractors (not named in Block 1) were named faster than former targets (named in Block 1). This negative repetition effect was not found when two words rather than two pictures were used or when a semantic categorization task was used with two pictures. From this we conclude that the effect reflects a process of mapping from a semantic representation to a name. Negative repetition was not found with a simultaneous selection cue, suggesting that it arose only when there was competition for name selection. It was also dependent on memory for previous acts of semantic naming. We propose that negative repetition reflects a form of speech monitoring that is applied when there is competition in the process of mapping from semantic to name representations.