Behavioural and imaging studies suggest that when humans mentally rehearse a familiar action they execute some of the same neural operations used during overt motor performance. Similarly, neural activation is present during action observation in many of the same brain regions normally used for performance, including premotor cortex. Here we present behavioural evidence that monkeys also engage in mental rehearsal during the observation of sensory events associated with a well-learned motor task. Furthermore, most task-related neurons in dorsal premotor cortex exhibit the same activity patterns during observation as during performance, even during an instructed-delay period before any actual observed motion. This activity might be a single-neuron correlate of covert mental rehearsal.