Pump loops designed to operate at temperatures and pressures to 320 deg C and 2000 psia are described. The loops were used to circulate uranyl sulfate solutions and aqueous thoria slurries in out-of-pile tests for the aqueous homogeneous reactor program. As this phase of the aqueous homogeneous reactor program was completed, the loops were converted for use in studies of materials of interest to other types of reactors. Solutions or slurries are circulated in the loops by means of a 100-gpm cannedmotor centrifugal pump. The loop piping is of 1- or 1 1/2-in. size and pressurizers of 4-in. pipe are used where required for steam pressurization. Two of the loops were constructed of commercially pure titanium while all others were of type 347 stainless steel. Some loops are hydraulically pressurized by means of a feed pump and letdown valve arrangement. The nominal volumes of the 15 loops, which were constructed varies between 10 and 30 liters, and each loop was designed to contain a large number of corrosion test specimens of various types for exposure under a wide range of conditions. Each loop test was usually of 200 to 500 hr duration, but some tests were operated continuously for more than 7,000 hr. Operating experience covering more than 350,000 hr of accumulated operating time in many hundreds of individual test runs is discussed. The pump loops described proved to be highly satisfactory systems in which to study reactor materials under a wide range of conditions which can be closely controlled to simulate those existing in nuclear reactors. (auth).