Flowering of many plant species is reduced or inhibited by exposure to high temperature. This reduction in flowering may be due to reduced photosynthesis, resulting in a reduced carbohydrate supply for floral initiation and/or development. Photosynthetic light response curves for heat-tolerant and heat-sensitive genotypes of Impatiens hawkeri Bull. and Viola x wittrockiana Gams. were developed. Plants were moved from a 20°C growth chamber to another chamber maintained at 35°C for varying lengths of time, and at different times during the day or night. High temperature exposures were: 2 h during the middle of the day (1200-1400 h) or night (2400-0200 h), 12 h during the day (0700-1900 h) or night (1900-0700 h), 24 h (beginning at 0700 h), or 72 h (beginning at 0700 h). After high temperature exposure, plants were returned to the 20°C chamber. High temperature exposures as brief as 2 h reduced net photosynthetic (Pn) rates of some cultivars the day after the exposure ended. A 72 h exposure to 35°C reduced Pn rate of all four cultivars the day after the exposure ended. Impatiens hawkeri photosynthetic rate was reduced to a greater extent than Viola x wittrockiana, and did not recover to the same photosynthetic rate as the untreated plants, as occurred for Viola.