In the past century, globally average daily minimum nighttime temperature increased at a faster rate than daily maximum temperature. The increase was 0.141 ºC per decade for maximum temperature and 0.204 ºC per decade for minimum temperature from 1950 to 2004. Annual mean maximum and minimum temperatures have increased by 0.41 ºC and 1.25 ºC, respectively; from 1979 to 2006 at the IRRI farm. Correlation analysis using historical data suggest that rice grain yield declined by 10% for each 1 ºC increase in growing-season minimum temperature. This statement needs to be confirmed experimentally. We designed temperature-controlled chambers in the field to study the effect of night temperature on the rice crop. The chambers were 6 x 3 x 2 m in length, width and height. Each chamber was equipped with an air conditioner. During daytime, the top and two sides of the chambers were opened so that the plants were exposed to the same environmental conditions as the ambient-grown plants. During nighttime, the chambers were closed and the air conditioner turned on to create about 2 ºC difference in night temperature between the two treatments. Temperature inside the chamber was fairly uniform. Relative humidity inside the chamber and at ambient was nearly 100% during the treatment. Carbon dioxide concentration inside the chamber was about 200 ppm higher than the ambient. Two varieties (IR72 and IR8) were grown inside the chamber and under ambient conditions. High night temperature reduced total dry weight and spikelet number per m² by 15-20%. The reduction was higher in IR8 than in IR72, suggesting that there is genotypic variation in sensitivity to warmer night temperature. Grain yield was not determined due to typhoon damage. Future studies will focus on understanding the fundamental effects of night temperature on physiological processes that govern crop growth and yield development.