Abstract: The chlorophyll fluorescence has been widely used to study the activity of the photosynthetic machinery. The first trifoliate leaf was used to study the effects of high temperature on the leaves using chlorophyll fluorescence method (Fv/Fm ratio). No effects were found on the ratio when leaves were stressed for 2 to 4 h at 35° C. After 4 h stress at 40°C, however, there was an approximately 50% decrease in the Fv/Fm ratio. The Fv/Fm ratio was more severely affected by heat stress at 42.5° C, but effects were greater in cultivars Mago-80 and Bragg. At 45°C there were approximately 80% decreases in the Fv/Fm ratios in all cultivars.