Different theories of syntactic ambiguity resolution argue for different sources of information determining initial parsing decisions (e.g., structurally defined parsing principles, lexically specific biases, or referential pragmatics). However, a "constraint-based" approach to syntactic ambiguity resolution proposes that both lexically specific biases and referential pragmatics are used in parallel by the comprehender. Analyses of text corpora, sentence fragment completions, and self-paced reading experiments were conducted to demonstrate that both local information (lexically specific biases) and contextual information (referential presupposition) contribute to the on-line resolution of prepositional phrase attachment ambiguities. There does not appear to be a role for purely structurally defined parsing principles (i.e., minimal attachment). Present and previous evidence is consistent with a developing framework in which multiple constraints (bottom-up and top-down) interact immediately to determine initial syntactic commitments.