An important property of geosynthetic clay liners (GCLs) is hydraulic conductivity. The authors describe and evaluate a new device, a GCL flowbox, that offers several advantages over large tanks or flexible-wall permeameters. The GCL flowbox can be used to test large-scale intact specimens, overlapped specimens with a full-width seam, and specimens subjected to environmental stresses such as freeze-thaw. A transparent, acrylic GCL flowbox was developed for testing at low normal stresses, and a metal version was developed for applying higher normal stresses. Results from the GCL flowbox compared favorably to results obtained with tanks and flexible-wall permeameters. It is concluded that the GCL flowbox provides a convenient and reliable testing methodology for verifying the hydraulic conductivity of both intact specimens and overlapped GCL panels.