In this experiment using the conditioned head-turn procedure, 18-month-old French-learning toddlers were trained to respond to either a target noun ("la balle"/"the ball") or a target verb ("je mange"/"I ea"t). They were then tested on target word recognition in two syntactic contexts: the target word was preceded either by a correct function word ("une balle""/a ball" or "on mange"/"they eat"), or by an incorrect function word, signaling a word from the other category (*"on balle"/"they ball" or *"une mange"/"a eat"). We showed that 18-month-olds exploit the syntactic context on-line to recognize the target word: verbs were recognized when preceded by a personal pronoun but not when preceded by a determiner and vice-versa for nouns. These results suggest that 18-month-olds already know noun and verb contexts. As a result, they might be able to exploit them to categorize unknown words and constrain their possible meaning (nouns typically refer to objects whereas verbs typically refer to actions).