Fusion of membranes is ubiquitous in life. It is essential for neurotransmitter and hormone release, intracellular vesicular trafficking, fertilization, and viral infection. SNARE proteins constitute a highly conserved minimal fusion machinery mediating intracellular membrane merger from slow fusion of large yeast vacuoles (minutes) to extremely fast neurotransmitter release (<1 ms). While membrane tension was suggested to inhibit fusion by suppressing dimpling of membranes by viral fusion proteins (Markosyan et al., Biophys J, 1999), it was suggested to promote fusion pore opening and dilation in other studies (Shillcock and Lipowsky, Nat Mater, 2005; Nikolaus et al., Biophys J, 2010; Warner and O'shaughnessy, Biophys J, 2012). Thus, membrane tension may affect distinct stages of the fusion process differentially. To resolve how tension affects fusion, we established a fusion assay in which membrane tension is precisely controlled. Our approach is based on a previously established bulk assay in which v-SNARE reconstituted small liposomes (vSUVs) fuse to t-SNARE containing giant unilamellar vesicles (tGUVs, ∼10-30 micrometers in diameter) (Malsam et al., EMBO J, 2012). Using a micropipette, a single GUV is picked up, whose membrane tension is controlled by the aspiration pressure. Another pipette is maneuvered nearby to puff a suspension of vSUVs. Fusion is monitored as an increase of the GUV tongue projection in the aspiration pipette whose position can be determined with sub-pixel resolution. Because the micromanipulation setup is mounted on a spinning disc confocal microscope, simultaneous monitoring of fluorescence from labeled membranes can also be used to probe vesicle docking and fusion. Our preliminary results show increasing membrane tension increases the fusion rate.