The behavior of iodine and xenon was studied during the experimental program of the Homogeneous Reactor Test to evaluate the various methods proposed for removal of these fission products to reduce the biological hazard of iodine and minimize xenon poisoning. The observed retention of both iodine and xenon in the reactor high-pressure system considerably altered design removal concepts. Only 10-25% of the iodine in the high-pressure system circulated with the fuel solution, and xenon, formed by decay of iodine adsorbed on the walls, was retained on the walls up to 10 hr before diffusing back into the circulating stream. The xenon poison fraction was found to be 0.010 at full power of 5 Mw.