Two experiments tested language switching effects with bilingual participants in a priming paradigm with masked primes (duration of 50ms in Experiment 1 and 100ms in Experiment 2). Participants had to monitor target words for animal names, and ERPs were recorded to critical (non-animal) words in L1 and L2 primed by unrelated words from the same or the other language. Both experiments revealed language priming (switching) effects that depended on target language. For target words in L1, most of the language switch effect appeared in the N400 ERP component, with L2 primes generating a more negative going wave than L1 primes. For L2 target words, on the other hand, the effects of a language switch appeared mainly in an earlier ERP component (N250) peaking at approximately 250ms post-target onset, and showing greater negativity following an L1 prime than an L2 prime. This is the first evidence for fast-acting language-switching effects occurring in the absence of overt task switching.