Energy-nutrient conversion ratios, as a measure of relative agricultural efficiency for Mexico, India, in the United States, and Japan between 1951 and 1974, were calculated for protein, B-vitamins, calcium, iron and food energy production. Estimates from the different nutrient ratios showed a range in percent change of production efficiency over the twenty-four year period. Indian estimates were the most consistent with only a 7 percent variability. Japan (23 percent), United States (42 percent), and Mexico (43 percent) each showed greater variability in their efficiency estimate ranges from aggregate energy-nutrient conversion ratios. Japan was the only country for which negative trends in efficiency were observed, as estimated from calcium and iron data. There did not appear to be any pattern in the relationship of one estimate to another in the countries studied. The variability of different nutrient efficiencies suggested that conventional methods of efficiency estimation based on a conversion ratio for a single nutrient may be inappropriate. There were significant differences in mean production efficiency estimates between countries but there was no uniformity among the determinations from different nutrients. The relative rank of efficiency for the countries changed for each of the efficiency estimates, except B-vitamins, over the time period studied. Diminishing returns in energy efficiency occurred as useful energy inputs increased. In addition, individual crops displayed tendencies toward diminishing returns. Plant foods were shown to be more efficient than animal sources of food. Efficiency increases appear to be related to increased use of effective energy inputs such as fertilizers.