In the present study, the significance of category-related brain activations as an index of semantic memory structure was assessed within a repetition-priming paradigm during a lexical decision task. The interpretation of category-related effects has been debated since previous studies observed category-related brain activity mainly in tasks requiring explicit semantic categorization. Furthermore, categories were frequently associated with behavioral performance differences, which could have influenced the pattern of brain activation. Event-related potentials (ERPs) to words denoting objects from artifactual (e.g., tools) and natural categories (e.g., animals) were recorded while subjects were presented with words and pseudowords as distracters, which were repeatedly presented. Category-related ERP differences emerged in the time window of the N400, an electrophysiological index of semantic processing, over occipito-parietal and fronto-central regions as well as in the following window of a late positive potential. Repetition priming modulated these category-related ERP effects whereas behavioral repetition priming (faster reactions to repeated words) was comparable for both categories. Differences in ERP repetition effects were specifically due to diminished category-related activity at repeated presentation. The present results show that category-related brain activation is not confined to tasks requiring explicit semantic categorization. Most importantly, the study demonstrates that category-related brain activation can be specifically modulated by repetition priming in the absence of corresponding behavioral performance differences. These findings therefore substantiate the significance of category-related brain activations as reflections of semantic memory structure and support the notion of multiple cortical semantic systems.