True in situ retorting of oil shale has been simulated in the laboratory in six experiments by igniting the common surfaces between two large blocks of oil shale while passing air between the blocks. The blocks were instrumented thoroughly and temperatures between the blocks and in the blocks are reported along with gas and oil analyses. The data allow estimation of combustion zone velocity to be 6.4 cm/hr, heating rates in the shale to be 19 /- 11C/min 2.5 cm from the fracture and 8 /- 3C 10 or 13 cm from the fracture. The amount of shale retorted was estimated to be 12% and about 1/3 of the oil produced was recovered. It was shown that movement of the combustion zone was diverted or stopped by condensed or coked oil, shale swelling, shale spallation, and shale fusion. It was also shown that dilution of the input air by carbon dioxide could be used to control the temperature in the combustion zone. It is suggested that a slower moving combustion zone may result in more efficient retorting of the shale.