Several fuel-loading concepts are proposed for high-temperature reactors of the pebble-bed type. A very promising one is the so-called OTTO (once through then out) loading scheme. Some of the intrinsic features of OTTO fuel loading are the axial nonsymmetrical power and neutron flux distribution with a pronounced maximum at the upper reactor core region. Since the neutron physics of OTTO cores will be very different from previous homogeneous fuel-loading schemes, detailed experimental and theoretical investigations of these objectives were performed at the critical facility KAHTER. Experimental and theoretical investigations have been carried out to determine critical masses, reaction rates, and control rod worths in the upper cavity and top reflector. Fast flux distributions in upper graphite reflectors were also measured to estimate graphite damage. The critical masses and keff’s are calculated using two- and three-dimensional code systems. The three-dimensional codes give keff values for the high-temperature gas-cooled reactor OTTO cores at zero burnup within a margin that is currently standard for these calculations. The agreement of measured and calculated reactivity worths of the top reflector rods is better than 2%.