A psychological refractory period (PRP) paradigm was used to isolate the locus of the picture-word interference effect along the chain of processes subtended in name production. Two stimuli were presented sequentially on each trial, separated by a varying stimulus onset asynchrony (SOA). The first stimulus, SI, was a tone that required a manual response. The second stimulus, S2, was a picture-word stimulus associated with picture naming. The distractor word was conceptually related to the picture on half of the trials, and unrelated in the other trials. A picture-word interference effect was found at long SOA, but not at short SOA. Such underadditive interaction between SOA and semantic relatedness suggests strongly that the locus of the picture-word interference effect is functionally earlier than the PRP effect locus. The results are discussed in relation to models of word production suggesting the involvement of central mechanisms in the selection of lexical output.