Results from a long-term field study indicate that alfalfa (Medicago sativa L.) plants died primarily in summer, especially in plots provided P but not K fertilizer. Our objective was to investigate the interaction between P and K nutrition and high temperature stress tolerance of alfalfa. A randomized complete-block design of four fertility treatments [400 kg per ha K, 75 kg per ha P, or no supplemental P or K] and two cultivars [CUF 101 (non-dormant) and 5454 (fall-dormant)] was replicated five times within each of three growth chamber regimes [high (35/30°C), optimum (25/20°C), or cool (15/10°C) day/night)]. Plants were established in low fertility soil in the greenhouse for two defoliation-regrowth cycles, and one-third of the plants were sampled prior to placing remaining pots into the contrasting temperature regimes. Half of the plants in each temperature regime were sampled at the end of 30 days, and remaining plants were defoliated and returned to the greenhouse where they were sampled after 30-d of recovery. At sampling, shoots were removed, counted, dried and weighed. Taproots were washed free of soil, the uppermost 5 cm finely chopped, and frozen in liquid N. Remaining root tissues were lyophilized, ground, and analyzed for sugar, starch, protein and amino-N concentrations. Herbage tissues were ground and analyzed for P and K concentrations. Preliminary results indicate significant effects of both temperature and P and K fertility on taproot sugar, starch, protein and amino-N concentrations. Cultivar effects were significant for taproot starch and amino-N concentrations. Additional analyses will explore the interaction between P and/or K fertility and high temperature stress on alfalfa in a way that permits us to understand how these nutrients enhance alfalfa growth and persistence in summer.