Carbon dioxide concentration during the dark is often not controlled in investigations of plant growth or in studies of plant responses to elevated [CO_2] in the field. In this work we have investigated whether [CO_2] at night significantly affected the growth of soybean seedlings, and whether this varied with the magnitude of the day/night temperature variation. Plants were grown under day/night [CO_2]s of 350/350, 350/700, 700/700 and 700/350μmol mol^<-1> [CO_2] in controlled-environment chambers under four temperature regimes. Field studies were also conducted using semi-open chambers in which plants were fumigated with air at the ambient [CO_2] or with air at ambient +350μmol mol^<-1> [CO_2] applied only during the day, only at night, or for 24h per day. When the day [CO_2] was low, elevated [CO_2] at night decreased plant leaf area and dry mass with constant day/night temperatures, but had no effect when there was a diurnal change in temperature. With high [CO_2] in the day, elevated [CO_2] at night decreased plant leaf area and dry mass when the diurnal change in temperature was small, but increased these parameters when the diurnal change in temperature was larger. The data for the field was similar to that obtained in the controlled-environment chambers in the regimes with the larger diurnal temperature changes. No effect of elevated [CO_2] on leaf area or biomass was observed in the field when the elevated [CO_2] treatment was applied only in the daytime. Additionally, the results indicate that a build up of [CO_2] at night in controlled-environment chambers or in the field can significantly reduce plant biomass under some temperature regimes for plants grown at low [CO_2] in the day time. The results indicate the usefulness of controlling [CO_2] at night in studies of plant growth.