In order to study the impact of a decline of leaf internal CO(2) molar ratio on nitrate reductase (NR) and sucrose-phosphate synthase (SPS) activities, leaves of wheat (Triticum durum) were submitted to different treatments: slow or rapid dehydration and decline in ambient CO(2) concentration and abscisic acid (ABA) supply. In agreement with the literature, NR activity of slowly dehydrated leaves was inhibited by about 50% when net CO(2) assimilation (A(n)) decreased by 45%. NR activity of stressed leaves kept 4 h in air containing 5% CO(2) or after 2 d of re-watering was only partially restored. NR activity was slightly dependent on ambient CO(2) molar ratio, declining by 30% when non-stressed leaves were kept in CO(2)-free air for 4 h. The decline of NR activity after ABA supply (through the transpiration stream) and after rapid dehydration of non-stressed leaves was comparable with the decrease observed under low CO(2) treatment. Overall, these data suggest that a drought-induced decrease of the leaf internal CO(2) concentration is only part of the signal triggering the decline of NR activity. In disagreement with most of the literature, SPS activity increased during slow dehydration, being stimulated by 30% when A(n) declined by 40%. SPS activity of stressed leaves kept 4 h in air containing 5% CO(2) or 2 d after re-watering was slightly increased or unchanged, respectively. By contrast to NR activity, SPS activity of well-hydrated leaves was hardly affected by low CO(2). Increased SPS activity was mimicked, in non-stressed leaves, by a rapid dehydration within 4 h and by ABA fed through the transpiration stream. In durum wheat, the increase in SPS activity could be linked to ABA-based signalling during a drought stress.