Neuropsychological impairments of English past tense processing inform a key debate in cognitive neuroscience concerning the nature of mental mechanisms. Dual-route accounts claim that regular past tense comprehension deficits reflect a specific impairment of morphological decomposition (e.g., jump + ed), disrupting the automatic comprehension of word meaning accessed via the verb stem (e.g., jump). Single-mechanism accounts claim that the deficits reflect a general phonological impairment that affects perception of regular past tense offsets but which might preserve normal activation of verb semantics. We tested four patients with regular past tense deficits and matched controls, using a paired auditory semantic priming/lexical decision task with three conditions: uninflected verbs (hope/wish), regular past tense primes (blamed/accuse), and irregular past tense primes (shook/tremble). Both groups showed significant priming for verbs with simple morphophonology (uninflected verbs and irregular past tenses) but the patients showed no priming for verbs with complex morphophonology (regular past tenses) in contrast to controls. The findings suggest that the patients are delayed in activating the meaning of verbs if a regular past tense affix is appended, consistent with a dual-route account of their deficit.