The present event-related potential (ERP) study was aimed at testing whether form-function mappings can differently affect sentence comprehension in early bilinguals with a range of linguistic profiles. Basque–Spanish and Spanish–Basque early bilinguals were presented with Spanish sentences with article-noun gender agreement violations. The gender of the target noun could be retrieved based on the word-form (i.e. transparent nouns) or only on a lexical representation (i.e. opaque nouns). While Basque-dominant bilinguals showed an impact of gender-to-ending consistency on agreement computation, Spanish-dominant bilinguals’ agreement processing was not affected by form-function mappings. A multiple regression analysis on early ERP responses from all participants showed that the more Spanish was produced on a daily basis, the easier the detection of gender violation for opaque nouns. The present results suggest that the strength of the lexical representation of gender is not fixed and can change depending on the linguistic habits of early bilinguals.