Technical Abstract: High surface temperatures are common to soils during periods of drought and when fields receive limited irrigation. Seedlings frequently experience these high temperatures during emergence and early seedling establishment in many regions of the world. The effects of a high temperature exposure on subsequent chlorophyll accumulation was determined in soybean (Glycine max) seedlings. Etiolated seedlings and excised cotyledons received a 30 min challenge temperature between 44 and 56 degrees C. Following the high temperature challenge in the dark, they were returned to optimal temperatures for chlorophyll and exposed to continuous light. The results show that chlorophyll accumulation was prevented by challenge temperatures of 50 degrees C and above in soybean cotyledons. Pre-incubation of the etiolated soybean cotyledons at 38 degrees C or 40 degrees C prior to the 50 degree C challenge provided maximum protection against high temperature injury. The time course for inducing acquired thermotolerance was dependen upon the tissue age (number of days following germination), as was the level of acquired thermotolerance. The results of this study demonstrate an inducible protection system and suggest that the temperature sensitivity of chlorophyll can be used as a sensitive bioassay for evaluating thermotolerance plants.