The effect of heat stress on photosynthetic electron transport was investigated in thylakoids isolated from the wheat (Triticum aestivum L.) varieties APU (Finland) and K65 (India) grown under both cool (13 °C day, 10 °C night) and warm (30 °C day, 25 °C night) regimes which gave rise to varietal differences in photosynthetic temperature acclimation. The responses of the uncoupled activities of both whole-chain electron transport and photosystem II to heat stress were similar. Both activities exhibited higher rates in thylakoids isolated from warm-grown plants and were more resistant to high temperature pretreatment than in those isolated from cool-grown plants, but varietal differences were not observed.Uncoupled photosystem I activity driven by either reduced 2, 6-dichlorophenol indophenol (DCPIPH₂) or N,N,N',N'-tetramethyl-p-phenylene diamine (TMPDH₂) showed a stimulation following high temperature pretreatment which was more marked in thylakoids isolated from warm-grown plants, followed by inhibition at extreme high temperatures. This stimulation was due largely to an increase in V_max but did not occur when reduced diaminodurene, which is highly lipophilic, was used as the electron donor. It appears that stimulation of PS I activity may involve increased accessibility of some artificial electron donors to the native acceptor sites within the thylakoid membrane in a process which is influenced by growth temperature.