Potato cultivars were grown in a glasshouse and plants periodically transferred to a growth chamber. When the ambient temperature in the growth chamber was raised from 15 to 40°C in steps of 5°C per hour, net photosynthetic rate decreased at temperatures above 20°C. At 40°C the rate was 37% of the rate at 20°C. A greater decrease in net photosynthetic rate occurred with plants of the cultivar Up-to-Date than with cultivars R100 and BP13. Low values of leaf diffusive resistance were recorded and changes in photosynthetic rate could not be explained by changes in this factor. When a constant ambient air temperature of 20°C was maintained while soil temperature was increased, net photosynthetic rate decreased.