The hypothesis was tested that visuospatial mental imagery relies on processing in the posterior parietal lobe. Using repetitive transcranial magnetic stimulation (rTMS) in a cross-over, sham-controlled design, we compared involvement of right posterior parietal cortex with primary visual cortex. Subjects received rTMS over the parietal and occipital cortices during 20 min, after which they performed a behaviorally controlled visuospatial mental imagery task. Performance deteriorated significantly after rTMS over the parietal, but not occipital, cortex. These data support a causal link between parietal activation and top-down spatial processing.