Positron emission tomography (PET) was used to determine the effect of working memory and speed of sentence processing on regional cerebral blood flow (rCBF) during syntactic processing in sentence comprehension. PET activity associated with making plausibility judgments about syntactically more complex subject-object (SO) sentences (e.g., The juice that the child spilled stained the rug) was compared to that associated with making judgments about synonymous syntactically simpler object-subject (OS) sentences (e.g., The child spilled the juice that stained the rug). Two groups of nine subjects differing in working memory and matched for speed of sentence processing both showed increases in rCBF in lateral posteroinferior frontal lobe bilaterally. The subjects were reclassified to form two groups of eight subjects who were matched for working memory but who differed in speed of sentence processing. Fast-performing subjects activated lateral posteroinferior frontal lobe bilaterally and slow-performing subjects showed activation of left superior temporal lobe. The results indicate that rCBF responses to syntactic comprehension tasks vary as a function of speed of sentence processing but not as a function of working memory.