The processes of pollen grain development and germination depend on the uptake and metabolism of pollen sugars. In pepper (Capsicum annuum L.), initial sugar metabolism includes sucrose hydrolysis by invertase and subsequent phosphorylation of glucose and fructose by hexose kinases. The main objective of this study was to investigate changes in fructokinase (EC 2.7.1.4) and hexokinase (EC.2.7.1.1) activities in pepper flowers during their development, and to study the possible roles of these enzymes in determining pollen germination capacity under high temperature and under CO(2) enrichment, previously shown to modify sugar concentrations in pepper pollen (Aloni et al., 2001 Physiologia Plantarum 112: 505-512). Fructokinase (FK) activity was predominant in pepper pollen, and increased during pollen maturation. Pollen hexokinase (HK) activity was low and did not change throughout pollen development. High-temperature treatment (day/night, 32/26 degrees C) of pepper plants reduced the percentage of pollen that germinated compared with that under normal temperatures (26/22 degrees C), and concomitantly reduced the activity of FK in mature pollen. High temperature also reduced FK and HK activity in the anther. Under high ambient CO(2) (800 micro l l(-1)) pollen FK activity was enhanced. The results suggest that pollen and anther FK may play a role in the regulation of pollen germination, possibly by providing fructose-6-phosphate for glycolysis, or through conversion to UDP-glucose (UDPG) to support the biosynthesis of cell wall material for pollen tube growth. High temperature stress and CO(2) enrichment may influence pollen germination capacity by affecting these pathways.