One hundred thirty-three pairs of precentral neurons were recorded simultaneously from four Macaca mulatta monkeys during participation in an operant conditioning paradigm that required one of the two units to be fired tonically within a 30- to 60-ms interspike interval (ISI) range. At the end of each 5-min behavioral period, a correlation between the two units' firing rate fluctuations was computed. These correlations were used as a measure of synchrony between units. For the majority of unit pairs, the unit for which reinforcement was contingent upon could be controlled independently from the activity of the simultaneously recorded unit. In 75% of experiments the synchrony between units decreased during the operant periods in comparison to the time-out periods. In only 5% of the experiments did unit pairs consistently exhibit increased synchrony during operant periods compared with time-out periods. If the monkey developed EEG signs of drowsiness during time-out periods, the synchrony between units became greater.