Single-unit recording in monkeys and functional imaging of the human frontal lobe indicate that the supplementary eye field (SEF) and the frontal eye field (FEF) are involved in ocular decision making. To test whether these structures have distinct roles in decision making, single-neuron activity was recorded from each structure while monkeys executed an ocular go/nogo task. The task rule is to pursue a moving target if it intersects a visible square or "go zone." We found that most SEF neurons showed differential go/nogo activity during the delay period, before the target intersected the go zone (delay period), whereas most FEF neurons did so after target intersection, during the period in which the movement was executed (movement period). Choice probability (CP) for SEF neurons was high in the delay period but decreased in the movement period, whereas for FEF neurons it was low in the delay period and increased in the movement period. Directional selectivity of SEF neurons was low throughout the trial, whereas that of FEF neurons was highest in the delay period, decreasing later in the trial. Increasing task difficulty led to later discrimination between go and nogo in both structures and lower CP in the SEF, but it did not affect CP in the FEF. The results suggest that the SEF interprets the task rule early but is less involved in executing the motor decision than is the FEF and that these two areas collaborate dynamically to execute ocular decisions.