Interference or conflict tasks are commonly used to investigate frontal lobe based mechanisms for interference control and resolution of response conflict. However, increased susceptibility to interference, as in Parkinsonís disease, may be significantly influenced by inhibitory dysfunction at a sensorimotor level. Evidence to support this view was obtained with masked response priming in an investigation with Parkinson patients (n=12), age- matched controls (n=12) and young controls (n=10). Using backward masking, covert activation of left or right hand responses was induced without subjects consciously perceiving the stimuli (right or left pointing arrows). The masked priming stimuli were followed by visible arrow stimuli, instructing for a left or right hand response, at a delay (ISI) of 0 or 100 ms. In young subjects, prime-target (in)compatibility effect had regular priming effects at ISI = 0 ms, but priming effects were reversed with ISI = 100 ms, consistent with an automatically invoked inhibition of prime-induced response activation (Eimer & Schlaghecken, 1998). The negative compatibility effect in young subjects (at ISI 100 ms), turned into a positive compatibility effect in Parkinson patients, while age-matched controls produced intermediate values. LRP recordings allowed a separation of age-related changes in performance and an inhibitory deficit in Parkinsonís disease.