In the present study we investigated, whether the mechanisms of integrating word meaning into semantic context as indexed by the N400 are already working during early language acquisition. Using a picture-word-matching design we recorded the ERP response of 12- and 19-month-old infants on slowly spoken basic level words. During the experimental session infants were looking at sequentially presented pictures while being acoustically presented with words which were either congruous or incongruous to the picture content. Infants at an age of 19 month show a broadly distributed long lasting semantic incongruity effect in their ERP when comparing the response on incongruous with that of congruous words. This infant N400 is preceded by an early phonological-lexical priming effect suggesting that infants at that age already create lexical expectations from the picture content. The same effect of facilitating early phonological processing can also be observed in the younger infants. From that we conclude that the semantic content of a picture primes an associated phonological-lexical representation even in 12-month-old infants. Infants at that age, however, do not display a N400-like semantic incongruity effect suggesting that, in one-year-old infants, phonological representations do not activate specific semantic representations sufficient to initiate a N400 response. Maturation of the N400, thus, occurs during the first half of the second year of life.