Some effects of a simulated overburden pressure on oil shale during retorting are presented. The results are applicable to underground retorting in systems where there is no stress relief. The range of pressures investigated was 25-2,500 psig at retorting temperatures up to 1,000$F. The effects of the simulated overburden pressure on the degree of thermal fracturing and exfoliation, the induced permeability and porosity, bulk volume changes, the effective retorting temperature, carbonate decomposition, and thermal conductivity were investigated. It was found that no visible fracturing or exfoliation occurs in oil shales retorted under confining stresses of 100 psig or more. Pore structure, however, is created by removal of oil and water, decomposition of carbonates and the creation of microscopic expansion cracks. The induced porosity is independent of overburden pressure, whereas in beds lying perpendicular to the maximum principal stress, the induced permeability is pressure-dependent. (13 refs.)