NREL has fabricated a parabolic trough receiver thermal loss test stand to quantify parabolic receiver off-sun steadystate heat loss. At an operating temperature of 400°C, measurements on Solel UVAC2 and Schott PTR70 receivers suggest off-sun thermal losses of approximately 370 W/m receiver length. For comparison, a receiver from the field with hydrogen in its annulus loses approximately 1000 W/m receiver length. The UVAC2 heat loss results agree within measurement uncertainty to previously published data, while the PTR70 results are somewhat higher than previously published data. The sensitivity of several receiver performance parameters is considered and it is concluded that differences in indoor and outdoor testing cannot account for the difference in PTR70 thermal loss results.