Neuropsychological research investigating mental grammar and lexicon has largely been based on the processing of regular and irregular inflection. Past tense inflection of regular verbs is assumed to be generated by a syntactic rule (e.g., show-ed ), whereas irregular verbs consist of rather unsystematic alternations (e.g., caught ) represented as lexical entries. Recent morphological accounts, however, hold that irregular inflection is not entirely rule-free but relies on morphological principles. These subregularities are computed by the syntactic system. We tested this latter hypothesis by examining alternations of irregular German verbs as well as pseudowords using ERPs. Participants read series of irregular verb inflection including present tense, past participle, and past tense forms embedded in minimal syntactic contexts. The critical past tense form was correct (e.g., er sang [he sang]) or incorrect by being either partially consistent (e.g., * er sung [*he sung]) or inconsistent (e.g., * er sing [*he sing]) with the proposed morphological principles. Correspondingly, in a second experimental block, pseudowords (e.g., tang /* tung /* ting ) were presented. ERPs for real words revealed a biphasic ERP pattern consisting of a negativity and P600 for both incorrect forms in comparison to the correct equivalents. Most interestingly, the P600 amplitude for the incorrect forms was gradually modulated by the type of anomaly with medium amplitude for consistent past tense forms and largest amplitude for inconsistent past tense forms. ERPs for pseudoword past tense forms showed a similar gradual modulation of N400. The findings support the assumption that irregular verbs are processed by rule-based mechanisms because of subregularities of their past tense inflection.