The plasticity of the adult memory network for integrating novel word forms (lexemes) was investigated with whole-head magnetoencephalography (MEG). We showed that spoken word forms of an (artificial) foreign language are integrated rapidly and successfully into existing lexical and conceptual memory networks. The new lexemes were learned in an untutored way, by pairing them frequently with one particular object (and thus meaning), and infrequently with 10 other objects (learned set). Other novel word forms were encountered just as often, but paired with many different objects (nonlearned set). Their impact on semantic memory was assessed with cross-modal priming, with novel word forms as primes and object pictures as targets. The MEG counterpart of the N400 (N400m) served as an indicator of a semantic (mis)match between words and pictures. Prior to learning, all novel words induced a pronounced N400m mismatch effect to the pictures. This component was strongly reduced after training for the learned novel lexemes only, and now closely resembled the brain's response to semantically related native-language words. This result cannot be explained by mere stimulus repetition or stimulus-stimulus association. Thus, learned novel words rapidly gained access to existing conceptual representations, as effectively as related native-language words. This association of novel lexemes and conceptual information happened fast and almost without effort. Neural networks mediating these integration processes were found within left temporal lobe, an area typically described as one of the main generators of the N400 response.