Annual additions to proven oil reserves are used to analyze yield per effort (YPE) between 1946 and 1989 in the lower 48 United States. YPE is the amount of oil added to reserves per foot of oil well drilled. The YPE data are used in a regression model that shows YPE is determined by long-term factors such as depletion and technical change, and short-term factors such as changes in the drilling rate, changes in the importance of revisions to oil reserves, and shifts between the intensive and extensive margin of exploration and development. When the short-term determinants of YPE are accounted for, YPE shows a significant exponential decline during the past 45 years. If the rate of drilling remains at the level it was in the late 1980s, the model projects about 21 billion bl of oil will be added to proven reserves from 1990 to 2010 in the lower 48 United States. Maintaining a drilling rate twice that of the late 1980s will add 29 billion bbl of oil to the proven reserves. Those projections suggest that proven oil reserves in the lower 48 United States will decrease sharply, with the magnitude of the decrease dependent also on the rate of oil production. The results also show that the expansion of exploration in the Gulf of Mexico did not alter significantly the overall decline in YPE for the entire lower 48 United States.