In an attempt to elucidate the mechanisms of directional selectivity in the neurons of the middle temporal visual area (MT) of macaque monkeys, we presented small numbers of sequentially flashed stimuli with various temporal and spatial intervals within the receptive field (RF) of direction-selective MT neurons. Experiments were performed using awake macaque monkeys trained to fixate on a set of short stationary lines. Stimuli were presented on a CRT screen under computer control. In two-flash experiments, responses to a test flash presented in the center of the RF were examined following a conditioning flash presented in various locations within the RF. Inhibition in the null direction was observed in about 78% of MT neurons, while facilitation was relatively weak in this group of neurons. In most of these neurons, the ranges of temporal and spatial intervals that produced directional selectivity in two-flash experiments were within half the values and double the values, respectively of those in multi-flash experiments. In the remaining 22% of direction-selective MT neurons, several flashed stimuli were necessary to produce directional selectivity. Most of these neurons showed facilitation in the preferred direction. It appears that the inhibitory mechanisms in the null direction are sufficiently strong to be induced by a single conditioning flash whereas the facilitatory mechanisms are weaker and several stimuli are required for production of the direction-selective response.