Plants of pepper (Capsicum amuum L.) were grown in controlled environment chambers at ambient (360 pmol mol"*) and fluctuating pulse-enriched CO2 concentrations (700 pmol mol"* daily average, ranging from 500 to 3500 pmol mol"* = ECO2) under two water regimes. A decrease in plant growth and yield together with frequent visual injuries was found in plants growing under ECO2. Root/shoot ratio was greater, chlorophyll concentration and respiration rates were lower, and stomatal conductance and relative importance of alternativě pathway respiration were higher under ECO2. The negative effects of ECO2 were more intense under high water availability. The symptoms produced by ECO2 were similar to those of resource limitation, and were alleviated with increased nutrient supply. Constant elevated CO2 concentrations (700 pmol mol"*) increased pepper production and did not produce any of the injuries described for this erratic ECO2 treatment. Thus, it is probably the erratic nátuře of the CO2 concentration and not the gas itself that was causing the injiuy.