Two rhesus monkeys were trained to control firing patterns of single neurons in parietal cortex (areas 1, 2, 3, 5, 7) using an operant task previously applied to the study of precentral units. Twenty-four of 56 (43%) postcentral cells were controlled in contrast to 71 of 136 (52%) precentral units from these and 4 other rhesus monkeys. In addition, monkeys were able to drive precentral units to more sustained tonic firing rates than they could parietal units. An analysis of interspike interval (ISI) distributions showed that, in contrast to precentral units with modal ISIs of 25-50 ms, 50% of parietal units have modal ISIs of 2 ms. Such short ISIs may account for fewer postcentral units reaching control criteria for this particular operant task. Other factors that may contribute to the reduced control of postcentral cells are discussed, particularly the more complex afferent connections to parietal units when compared to precentral pyramidal tract neurons. The data indirectly support conclusions from previous studies that imply that operant control of cortical units is peripherally mediated and does not primarily involve a 'central' or 'open loop' system.