OBJECTIVES: Through event-related potential (ERP) recording, to better understand the perceptive-motor slowness in adults with Down syndrome (DS); in particular, to assess whether motor preparation influences the speed of allocation of perceptual attention as reflected by the P3 latency. PATIENTS AND METHODS: ERPs were obtained in adults with and without DS through an auditory oddball paradigm under a passive and two active (simple vs. complex motor response) conditions. Reaction times (RTs) were recorded in the two active conditions. RESULTS: There was no influence of movement complexity on either RT or P3b latency in the control group. In the DS group, RT was delayed under simple vs. complex conditions whereas P3b latency was not affected. N2a and N2b were often missing in DS individuals. CONCLUSION: Motor preparation processes per se rather than interaction with perceptual attention could be defective in individuals with DS when the motor component of the response is minimal.