Chlorophyll a (Chl a) florescence induction kinetics from the minimum yield F₀ to the maximum yield F_m provide information on the filling up of the plastoquinone pool with reducing equivalents. In this paper, we have examined the effect of high temperature (above 40°C) on Chl a fluorescence rise kinetics starting from 40 μs (to 1 s) in pea leaves (Pisum sativum). The variable Chl a fluorescence is strongly quenched after heat treatment. With increasing temperature or the duration of heat treatment a typical O-J-I-P transient (Strasser et al. (1995) Photochem. Photobiol., 61, 32–42) is transformed into an O-K-J-I-P transient, with an additional rapid step called K detected in the 200–300 μs range. After prolonged heat treatment, the K-step becomes a dominant peak in the Chl a fluorescence transient followed by a large dip. We have investigated the origin and the possible interpretation of these changes by using NH₂OH which acts as an electron donor to PS II, and DCMU which is known to block the PS II electron transport chain by displacing Q_B. From the present data we propose that the appearance of this K-step is due to two effects: (1) inhibition of the water splitting system that leads to a much slowed turn over of the reduction of Q_A; (2) changes in the architecture of the antenna of PS II which affect the energy migration properties within the photosynthetic unit. The K-step can thus be used as an indicator of the heterogeneity of photosynthetic units and as an indicator for the physiological state of the photosynthetic sample.