This report documents test conditions and results from the last two tests in a series used to study yield losses caused by permeability contrasts in modified in situ oil shale retorts. Zones of contrasting permeability are created by loading particles of one size distribution into part of the retort and those of another size distribution into the remainder. Shale for tests S78 and S80 was loaded using identical shale grade, particle size, and bed geometry to create duplicate shale beds. Tests S78 and S79 were conducted using a 70% air and 30% steam mixture flowing at 1.25 standard cubic feet of gas per square foot of empty bed each minute. Test S80 was conducted with air flowing at 0.875 standard cubic feet per square foot per minute while injecting propane into the top of the high permeability zone to maintain 1600 to 1800/sup 0/F in the high permeability zone behind the retorting combustion front. The high temperature in the high permeability zone in test S80 increased resistance to flow by increasing the viscosity and reducing the density of gases flowing through that zone. The increased resistance reduced gas flow and slowed the retorting front advance rate in the high permeability zone while increasing the advance rate in the zone having low permeability. The overall effect was to cause a more uniform retorting front which improved sweep efficiency and increased oil yield. We found the use of external fuel in test S80 increased the sweep efficiency from 50% in test S78 to 80% and increased the oil yield from 16.5 to 32.2% of Fischer assay yield. The increase in oil and gas yield from test S80 over S78 resulted in a net energy ratio of 2.4 based on the gross calorific values of the increases in oil yield and gas yield compared to the propane consumption. 8 refs., 5 figs., 8 tabs.