The effect of CO2enrichment on the rate of photosynthesis and the water use efficiency (WUE) of young pepper and tomato plants was studied in the phytotron. A CO2level of 1000 ppm significantly increased the net assimilation rate in the upper foliage, while the increase was even more considerable in the lower layers of the canopy, with values of up to 100%. The 1500 ppm CO2level caused a further substantial increase in CO2assimilation and at least doubled (in tomato) or tripled (in pepper) the water use efficiency on a leaf area basis compared to the ambient values. Although the response in terms of photosynthesis and WUE was not variety-specific, there were differences between the pepper hybrids in the biomass components, exceeding 100% for the total biomass at the 1500 ppm CO2level. In tomato, however, there was no significant variation in the total biomass of the three hybrids investigated in this early phase of development at either CO2level.