The role of nutrients in regulating phytoplankton populations remains a problem of central importance in aquatic ecology. It is of particular interest during the period of summer stratification. Cayuga Lake was used in this study since it is a favorable environment for investigating nutrient effects on phytoplankton. It is strongly stratified in the summer, and both phosphate and silicate are reduced to low levels, whereas nitrate remains relatively abundant. During 1966, a series of nutrient enrichment experiments were performed. The effects of major nutrient elements on rates of 14-C uptake were tested in short-term experiments. During the summers of 1967 and 1968 similar enrichment experiments were continued and were described here. Water samples for experiments and for nutrient and phytoplankton analyses, were obtained from mid-lake between Portland Point and Taughannock Point for both years. A total of seven experiments were completed in 1967 and the results were summarized. The following summer the experiments were modified and twelve additional experiments were then performed. The results of all nineteen experiments were then summarized. A discussion and comparison of the experiments of 1967 and 1968 followed. (Author)