In this study, we investigated semantic context effects in language production with event-related brain potentials, extracted from the ongoing EEG recorded during overt speech production. We combined the picture-word interference paradigm and the semantic blocking paradigm to investigate the temporal dynamics and functional loci of semantic facilitation and interference effects. Objects were named in the context of semantically homogeneous blocks consisting of related objects and heterogeneous blocks consisting of unrelated objects. In each blocking condition, semantically related and unrelated distractor words were presented. Results show that classic patterns of semantically induced facilitation and interference effects in RTs can be directly related to ERP modulations located at temporal and frontal sites, starting at about 200 msec. Results also suggest that the processes associated with semantic facilitation and interference effects (i.e., conceptual and lexical processing) are highly interactive and coincide in time. Implications for the use of event-related brain potentials in speech production research and implications for current models of speech production are discussed.