1. Evidence from single-unit recordings suggests that neurons in the medial superior temporal visual area (MST) carry directional signals that influence psychophysical judgements of motion direction. We tested this hypothesis by electrically stimulating clusters of directionally selective neurons in MST (the dorsomedial subdivision, primarily) while rhesus monkeys performed a two-alternative, forced-choice direction discrimination task. 2. We performed forty-six microstimulation experiments on two rhesus monkeys. The visual stimuli were dynamic random dot patterns in which the strength of a coherent motion signal could be varied continuously about psychophysical threshold. The monkeys were rewarded for reporting correctly the direction of the coherent motion signal. Microstimulation was applied on half the trials, selected randomly, and the psychophysical data were analyzed to determine whether stimulation of MST neurons influenced the monkeys' choices. 3. Microstimulation influenced the monkeys' performance in a statistically significant manner in 67% of the experiments. In all but one of the significant experiments, microstimulation biased the monkeys' choices toward the direction of motion encoded by MST neurons at the stimulation site. Microstimulation had little effect on the slopes of the psychometric functions, suggesting that the stimulation-induced neural activity resembled a relatively pure motion "signal" rather than "noise." 4. Microstimulation exerted strong effects on the monkeys' behavior only when the visual stimulus was located within the multiunit receptive field measured at the stimulation site. This kind of spatial specificity has also been observed in the middle temporal visual area (MT), but receptive fields in MST are typically much larger than those in MT. Thus MST microstimulation effects are characterized by a coarser spatial scale: stimulation of a single site in MST can influence judgements over a much larger portion of the visual field than equivalent stimulation in MT. 5. Microstimulation was often most effective when visual stimuli were placed within a particularly responsive subregion of the receptive field (a "hot spot"). 6. The results show that MST neurons, like MT neurons, can strongly influence performance on a direction discrimination task. Whether MT and MST influence the decision process in parallel or in series remains to be determined.