Invertase, beta-fructofuranoside fructohydrolase (E.C. 3.2.1.26), catalyzes the hydrolysis of sucrose into fructose and glucose. This function has been implicated in providing carbon substrate for respiration. Invertase activity has also been implicated as important in plant responses to some stresses. High nighttime temperatures capable of decreasing rice (Oryza sativa L.) yield or quality can occur at seed set or early grain fill in some seasons in the southern U.S. rice growing regions. This study is evaluating (1) a hormonal stimulant of invertase activity, thidiazuron; (2) a chemical stimulant of invertase activity, concanavalin A (ConA) from Canavalia ensiformis; and (3) a chemical inhibitor of invertase activity, ammonium molybdate; to determine their influence on the activity of invertase in rice under heat stress during early grain fill. This study is being conducted at the Texas A&M Agricultural Research and Extension Center at Beaumont, Texas in the greenhouse. The plants' temperature is precisely controlled using an infrared heating system with power controller to maintain temperature at 30șC (±0.5șC) while allowing the plant to grow in a free-air environment. In the panicle, the plants exposed to heat but without chemical treatment and plants without heat treatment had higher sucrose cleaved * g DW tissue^-1 * hr^-1 than plants treated with ConA, thidiazuron, or ammonium molybdate. In the penultimate leaves, heat reduced the amount of sucrose cleaved relative to plants without chemical or heat treatments. Furthermore, thidiazuron and ConA increased sucrose cleaved above that due to the heat treatment to levels found comparable to and greater than, respectively, activities of plants with no heat treatment. Tissue differences in treatment responses will be discussed.