Oil spillage connected with federally supervised drilling and production activities has been a matter of wide public concern. In its supervision of mineral-resource development on the Outer Continental Shelf (OCS), the U.S. Geological Survey is responsible for the day-to-day inspection and monitoring of OCS oil and gas operations. During these activities, the U.S. Geological Survey records and investigates hydrocarbon discharges resulting from such operations. Beginning in 1971, all spills have been recorded, and a computer file has been maintained on all spills of 1 barrel or more. The total Gulf of Mexico OCS oil spillage recorded during January 1, 1971-December 31, 1975, amounted to 51,421 barrels. Production during that period amounted to 35,219 barrels per barrel spilled. In all, 5,857 spills were recorded, but 85.5 percent of the total spill volume was contributed by just 5 incidents. The environmental effect of these incidents apparently was minimal and of short duration. No spills of more than 50 barrels resulted from drilling operations during the period. The only spillage resulting from blowouts was caused by nondrilling incidents, including completion, production, and workover. The amount of oil discharged from spills of less than 50 barrels decreased by more than half between 1971 and 1975. The improvement reflects changes in the operating philosophy of the offshore industry, tightening of U.S. Geological Survey operating orders, and substantial increases in the inspection force. Most production-platform spills involve failures in the sump system, the separator system, or other hydrocarbon-handling equipment; improved sump-system designs and better high-low-level controls have reduced both the number and the volume of spills. Pipeline and pump spills also declined significantly, although the decline appears less attributable to revisions in OCS operating requirements. No operator consistently contributed a disproportionate amount of spillage. Most of the platforms having production spills had only one spill during the period. Of the 304 platforms having incidents, 255 had 2 spills or less and 188 had only 1 spill. Comparison of the volume spilled with the volume produced for Gulf of Mexico OCS operations during 1971-75 results in a spillage rate of 0.0028 percent. Spillage from tankers is estimated to be greater. The OCS drilling and production spillage rate is not expected to increase during exploration and development of the frontier areas.