In situ recovery technologies for tar sand and heavy oil are experimentally simulated at the Western Research Institute to develop a technological base for understanding in situ processing of these resources. Wide ranges of process parameters are tested in one-dimensional reactor tubes in which the tar sand or heavy oil sand is uniformly reconstituted to nearly the same permeability and porosity as the field resource. Three-dimensional process performance is evaluated by testing the most promising operating parameters in consolidated blocks of the field resource. A three-dimensional test of in situ combustion was conducted on a block of material from the Tar Sand Triangle. This test was the second such test conducted on the Tar Sand Triangle material and incorporated several design features to overcome problems identified in the first test. Although combustion was established in the block, the maintenance and advancement of the combustion front was not successful. The problem with sustaining combustion appears to be excessive heat loss to the surrounding refractory and the sweep gas. These heat losses reduce the temperature in the combustion zone and inhibit utilization of the oxygen in the injected gas. Poor oxygen utilization leads to lower combustion temperatures and less driving force for the combustion front. 6 refs., 7 figs.