Growth of autotrophically growing duck-weeds (Lemna gibba L., G1) was stimulated by sucrose. The rate of respiration increased when plants had been grown on sucrose (8.7 μmol O2 g(-1) fresh weight (FW) h(-1)) and was reduced after growth without sucrose in the dark or under longday conditions (2.5 μmol O2 g(-1) FW h(-1)). Photosynthesis was induced already by low light intensities (0.1 klx).Short-time application of glucose or sucrose stimulated respiration in proportion to the hexose uptake rate. Sucrose is probably not taken up as the disaccharide. The transported sugar species after addition of sucrose are its hexose moieties produced by the high activity of the cell wall invertase. Fructose stimulated to a lesser extent; mannitol induced no enhancement; 2-deoxyglucose slightly inhibited O2 uptake. After mild carbon starvation of the plants the uptake of glucose and 3-O-methylglucose proceeded without any lag phase, with similar saturation kinetics in both cases. The initial uptake rate at substrate saturation was 2.6 μmol glucose g(-1) FW h(-1) in the dark. Light stimulated hexose uptake by 2 to 3 times. The results show that Lemna gibba has an energy-dependent constitutive system for hexose uptake.