We present observations of Hi in the nearby interacting galaxies NGC 4490 and 4485 made with the VLA in both C and D arrays. The galaxies are embedded in an extensive envelope of neutral hydrogen which is elongated in a direction approximately perpendicular to the plane of NGC 4490, with an extent of about 56kpc. We argue that this distribution of neutral hydrogen can best be explained by a galactic-scale bipolar outflow of Hi driven by supernovae in NGC 4490. The flow from the disc appears to be reasonably well collimated and has probably persisted for approximately 6x10^8yr. The implications for galaxy evolution when such mass loss occurs are briefly discussed.