1. We studied the responsiveness of neurons in the primary motor cortex (MI) of monkeys (Macacafuscata) to electrical stimulation of the supplementary motor area (SMA), primary sensory cortex (SI), and the ventral subnucleus of the thalamus (VPLo) with chronically implanted electrodes. 2. All neurons examined in this study were characterized by their relation to a motor task performed by the animals. They responded to stimulation of the cortical or thalamic area with excitation from one area alone (n = 128) or from multiple areas (n = 84) of all combinations. In a majority of neurons, response latencies to both cortical and thalamic stimulation were within 5 ms. 3. A vast majority of neurons (80%) that were active during a preparatory period for forthcoming reaching movements were activated by SMA stimulation. They were activated only infrequently by SI or thalamic stimulation. 4. Movement-related neurons (active immediately before and during reaching movements) were activated by thalamic, SI, or SMA stimulation or by any combination of those stimuli. More than half of the movement-related neurons activated exclusively by either thalamic or SMA stimulation exhibited activity onset times earlier than those observed in the earliest muscles. By contrast, most movement-related neurons that responded only to SI stimulation were late in their activity onset. 5. These findings suggest that the SMA input to MI is important in developing a preparatory type of activity in MI, whereas the thalamus (VPLo) provides substantial inputs in movement execution. The roles played by inputs from SI and SMA in relation to motor execution are debatable and are discussed here with reference to previous reports.