One hundred twenty five pairs of neurons were recorded simultaneously from precentral cortex of Macaca mulatta monkeys during an operant conditioning task. At the end of 5-min behavioral periods, a cross-correlation histogram was generated to look for relationships between the firing of the two cortical neurons. Eighty four (67%) unit pairs showed a significant coincidence of firing within 1 ms of each other. This relationship occurred regardless of whether the units' firing rate fluctuations were correlated or not. These data imply that in the majority of cases, the two units are probably more related than previously reported.