Abstract.Elevated levels of CO2in the atmosphere are expected to affect plant performance and may alter global temperature patterns. Changes in mean air temperatures that might be induced by rising levels of CO2and other greenhouse gases could also be accompanied by increased variability in daily temperatures such that acute increases in air temperature may be more likely than at present. Consequently, we investigated whether plants grown in a CO2enriched atmosphere would be differently affected by a heat shock than plants grown at ambient CO2levels. Plants of a C3annual (Abutilon theophrasti), a C3annual crop (Sinapis alba) and a C4annual (Amaranthus retroflexus) were grown from seed in growth chambers under either 400 or 700cm3m−3CO2, and were fertilized with either a high or low nutrient regime. Young seedlings ofS. alba, as well as plants of all species in either the vegetative or reproductive phase of growth were exposed to a 4‐h heat shock in which the temperature was raised an additional 14–23°C (depending on plant age). Total biomass and reproductive biomass were examined to determine the effect of CO2, nutrient and heat shock treatments on plant performance. Heat shock, CO2, and nutrient treatments, all had some significant effects on plant performance, but plants from both CO2treatments responded similarly to heat shocks. We also found, as expected, that plants grown under high CO2had dramatically decreased tissue N concentrations relative to plants grown under ambient conditions. We predicted that high‐CO2‐grown plants would be more susceptible to a heat shock than ambient‐CO2‐grown plants, because the reduced N concentrations of high‐CO2grown plants could result in the reduced synthesis of heat shock proteins and reduced thermotolerance. Although we did not examine heat shock proteins, our results showed little relationship between plant nitrogen status and the ability of a plant to tolerate an acute increas