Cross-correlating the activity of neighboring motor cortex neurons recorded with independent electrodes in behaving monkeys has revealed synchronization peaks, largely due to common synaptic input. Corticomotoneuronal (CM) cells produced post-spike facilitation (PSF) of rectified forearm electromyograms (EMG); 15 cells synchronized with CM cells showed no PSF. Five pairs of CM cells with overlapping muscle fields exhibited similar synchrony peaks. The contribution of this synchrony to facilitation of common target muscles was assessed by two new methods: selective spike-triggered averaging and convolution. They showed that the PSF is composed predominantly of effects mediated by output of the triggering cell, but may include a broad, shallow component mediated by synchrony with other CM cells.