The precise role of frontal eye fields (FEF) in vision independent of their role in eye movements remains a matter of debate. One proposal is that the FEF exert top-down influences on the extrastriate visual cortex prior to eye movement preparation. Here we establish, by use of transcranial magnetic stimulation (TMS), that activity in the human FEFs has a direct effect on the sensitivity of extrastriate visual area MT/V5, and that the spatial organization of this top-down effect is lateralized in the human brain. We show that phosphene threshold-the TMS intensity required to elicit a visual perception-for MT/V5 stimulation changes as a function of the delay between the application of TMS over FEF and MT/V5. The effects were specific to the location and time of stimulation. Stimulation of FEF 20-40 ms prior to stimulation of MT/V5 decreased the intensity of MT/V5 stimulation required to elicit phosphenes: TMS of the right FEF changed the sensitivity of left and right MT/V5 whereas TMS of the left FEF changed the sensitivity only of the left MT/V5. Thus, the sensitivity of human extrastriate cortex is modulated by activity in the FEF.