Results of the first 150 days of operation of an in situ oil shale retorting experiment near Rock Springs, Wyoming, are presented. A zone of oil shale located in the Tipton Member of the Green River Formation was retorted. The target zone was 137 feet deep, 40 feet thick, and assays 21.3 gallons of oil per ton of shale. A section 70-feet square with eight peripheral production wells was hydraulically fractured to produce three horizontal fractures. Slurry-type explosive was forced into the fractures and detonated to produce a bed of oil shale rubble.