The dorsal medial superior temporal area (MSTd) in the extrastriate visual cortex is thought to play an important role in heading perception because neurons in this area are tuned to both optic flow and vestibular signals. MSTd neurons also show significant correlations with perceptual judgments during a fine heading direction discrimination task. To test for a causal link with heading perception, we used microstimulation and reversible inactivation techniques to artificially perturb MSTd activity while monitoring behavioral performance. Electrical microstimulation significantly biased monkeys' heading percepts based on optic flow, but did not significantly impact vestibular heading judgments. The latter result may be due to the fact that vestibular heading preferences in MSTd are more weakly clustered than visual preferences and multiunit tuning for vestibular stimuli is weak. Reversible chemical inactivation, however, increased behavioral thresholds when heading judgments were based on either optic flow or vestibular cues, although the magnitude of the effects was substantially stronger for optic flow. Behavioral deficits in a combined visual/vestibular stimulus condition were intermediate between the single-cue effects. Despite deficits in discrimination thresholds, animals were able to combine visual and vestibular cues near optimally, even after large bilateral muscimol injections into MSTd. Simulations show that the overall pattern of results following inactivation is consistent with a mixture of contributions from MSTd and other areas with vestibular-dominant tuning for heading. Our results support a causal link between MSTd neurons and multisensory heading perception but suggest that other multisensory brain areas also contribute.