Suspensions of UO2 in NaK alloy are being studied as possible reactor fuels. Two loops have so far been constructed of one-half inch stainless steel tubing with a test volume of approximately 150 cc. The Mark I loop was operated for 400 hours between 450 and 600°C, while circulating a slurry of 4.3 volume % UO2 (36.0 weight per cent). The radioactive monitor indicated that the slurry was uniformly suspended at a flow rate of 2 ft/sec. It was found that above 500°C the UO2 dropped out of suspension, but was immediately resuspended when the temperature dropped below the critical value of 500°C. The effect was reversible and could not be eliminated even at the fastest flows. The Mark II loop incorporated a density measuring device in order to correlate the radioactivity monitor with the density of the slurry. It was found, as previously suspected, that when the counts reached maximum value the density also was a maximum, at the calculated value. One gram of powdered uranium metal added to the loop with the UO2 completely eliminated the settling above 500°C observed in the Mark I loop. There was no evidence for corrosion or erosion of the loop, and the particle size of UO2 was practically unchanged after operation at the high temperature. The UO2 could be resuspended immediately after prolonged settling.