As we move through the environment, the pattern of visual motion on the retina provides rich information about our movement through the scene. Human subjects can use this information, often termed "optic flow", to accurately estimate their direction of self movement (heading) from relatively sparse displays. Physiological observations on the motion-sensitive areas of monkey visual cortex suggest that the medial superior temporal area (MST) is well suited for the analysis of optic flow information. To test whether MST is involved in extracting heading from optic flow, we perturbed its activity in monkeys trained on a heading discrimination task. Electrical microstimulation of MST frequently biased the monkeys' decisions about their heading, and these induced biases were often quite large. This result suggests that MST has a direct role in the perception of heading from optic flow.