During fiscal year 1987, emphasis in the Hot Dry Rock Geothermal Energy Development Program was on preparations for a " Long-Term Flow Test " of the "Phase II" or "Engineering" hot dry rock energy system at Fenton Hill, New Mexico. A successful 30 day flow test of the system during FY86 indicated that such a system would produce heat at at temperature and rate that could support operation of a commercial electrical power plant. However, it did not answer certain questions basic to the economics of long-term operation, including the rate of depletion of the thermal reservoir, the rate of water loss from the system, and the possibility of operating problems during extended continuous operation. Preparations for a one year flow test of the system to answer these and more fundamental questions concerning hot dry rock systems were made in FY87: design of the required surface facilities; procurement and installation of some of their components; development and testing of slim line logging tools for use through small-diameter production tubing; research on temperature-sensitive reactive chemical tracers to monitor thermal depletion of the reservoir; and computer simulations of the 30-day test, extended to modeling the planned Long-Term Flow Test