Two wheat (Triticum aestivum L.) cultivars, Mustang and Sturdy, have been shown to exhibit genetic diversity in acquired thermal tolerance following exposure to 37-degrees-C. The present study used chlorophyll fluorescence to investigate (1) the sensitivity of photosystem II (PS II) to 37-degrees-C treatments, and (2) the recovery of PS II variable fluorescence at 22-degrees-C following heat treatment. The purpose was to determine if the observed diversity in cellular thermotolerance was related to differential sensitivity of PS II to elevated temperatures. PS II activity, monitored by chlorophyll fluorescence transients, was inhibited by 40-50% in both Mustang and Sturdy by a 37-degrees-C treatment for 30 min. Continued heat treatment of the seedlings for up to 5 hr at 37-degrees-C inhibited PS II fluorescence by approximately 80%. Returning the heat treated seedlings to 22-degrees-C for 24 hr resulted in the complete recovery of PS II variable fluorescence. The time course of the recovery of PS II variable fluorescence at 22-degrees-C after the heat treatment showed a complete return to normal levels following a 1, 4 or 16 hr pretreatment at 37-degrees-C. PS II variable fluorescence did not completely recover following 24, 48 or 72 hr of heat treatment. No differences in either the sensitivity or recovery of PS II function were observed in these two cultivars of wheat. The results of this study suggest that the observed diversity in acquired thermal tolerance in these wheat cultivars is not associated with changes in the thermal sensitivity of PS II.