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Mediterranean vegetation is frequently subjected to the heat waves with temperature peaks exceeding 45 degrees C. While the effects of drought on assimilates allocation are deeply studied, little information is available regarding the effects of the acute heat stress on this aspect of the plant physiology. We evaluated the effects of acute heat pulses on plant C internal allocation in Eucalyptus camaldulensis saplings and its transfer into the soil. Shoots of Eucalyptus were subjected to an increase of the air temperatures up to 45 degrees C. Allocation of C was inferred after pulse labelling of heated and non-heated shoots with (CO2)-C-13 . Heat pulse lowered the photosynthesis and almost tripled the C losses from shoots through respiration. Assimilated C-13 was preferentially allocated to leaf soluble sugar fractions rather than to starch. Heat pulse interrupted C-13 allocation to the trunk, roots and soil. C-13 allocation to new non labelled leaves was detected in some replicate plants, independently of treatment, suggesting that this process is sink-driven. Although, shoots subjected to the heat pulse modified allocation strategy, "saving" assimilated C-13 at a leaf level, the rate of C-13 involvement in shoot respiration was restricted by photosynthetic rates measured during the labelling. The results suggest that photosynthetic and non-photosynthetic organs may be constrained to rely on C reserves under elevated temperatures with further negative consequences for C balance in case the duration and frequency of the heat waves will increase as projected by climate change.