We present a new technique for studying the activation of semantic and phonological codes in speech planning using event-related brain potentials (ERPs) that extend a well-established behavioral procedure from speech production research. It combines a delayed picture-naming task with a priming procedure. While participants prepared the production of a depicted object's name, they heard an auditory target word. If the prepared picture name and the target word were semantically or phonologically related, the ERP waveform to the target word tended less towards the negative when compared to an unrelated control. These effects were widely distributed. By contrast, if participants performed a nonlinguistic task on the depicted object (natural size judgment), the semantic effect was still obtained while the phonological effect disappeared. This suggests that the former effect indexes semantic activation involved in object processing while the latter effect indexes word-form activation specific to lexical processing. The data are discussed in the context of models of lexical access in speech production.