The United States has invested substantial resources in agricultural research since the Morrill and the Hatch Acts. These investments have made American agriculture one of the most productive in the world. Several studies have evaluated U.S. agricultural research. However, few of these studies have attempted to assess the decline in agricultural productivity that would have resulted in the absence of agricultural research. The purpose of this dissertation is to measure the magnitude of agricultural maintenance research currently or recently undertaken to forestall such productivity declines. A two part procedure was used to evaluate the importance of maintenance research in U.S. agriculture. First, questionnaires were mailed to agricultural scientists at the state agricultural experiment stations. The information sought from the scientists included examples of maintenance research and research depreciation, their annual research budgets, and the percentage of their research efforts devoted to maintenance research. The second part of the procedure was to estimate a profit function model to assess the importance of research depreciation in U.S. agriculture and to test the overall length and shape of the research lag. Duality theory was used to obtain the output supply (foodgrains, feedgrains, other crops, hay, livestock, and poultry), input demand (feed, fertilizer, fuel, and labor) equations. The fixed factors included were land, research, extension, education, capital, and breeding stock. Secondary data, from various sources, were used to estimate the equations. The results from the responses to the questionnaire indicate that, on average, the United States devotes roughly a third of total agricultural production research to maintenance research. In addition, there are significant differences in maintenance research among individual commodities. The results from the output supply equations indicated that the impact of agricultural research on agricultural output rises and then declines for some commodities. Also, research depreciation occurs for some agricultural commodities and maintenance research may be required to prevent productivity from declining. However, the results were for the most part, not statistically significant, reducing the strength of the conclusion that can be drawn. / Ph. D.