Recent studies provide evidence for an interaction between a declarative memory system, dependent on the medial temporal lobe (MTL), and a habit memory system, dependent on the basal ganglia. Using functional MRI, the authors studied this interaction when 1 system was compromised by neurological disease. Neural activity when performing a habit-learning task was compared in normal controls and subjects with Parkinson's disease (PD). Patients with PD showed less activation in the caudate nucleus and greater activation in a region of prefrontal cortex that has been associated with explicit memory retrieval. Patients with PD also showed activation of the MTL during the weather-prediction task. These findings are consistent with an interaction between memory systems of the MTL and the striatum.