Atmospheric CO2 concentrations have been increasing from about 280ppm to 400ppm from the pre-industrial era until now. If intraspecific variability in the response to elevated CO2 (eCO2) can be found, then it should be possible to select for greater responsiveness in crop breeding programs. Our experiment aimed to determine the effects of eCO2 on the yield, biomass, leaf and grain nitrogen content of a range of field pea (Pisum sativum L.) cultivars subjected to rainfed and supplemental irrigation conditions. Plants were grown under Free Air CO2 Enrichment (FACE) at the Australian Grains FACE facility in Horsham, Victoria, Australia under eCO2 (550ppm) or at ambient CO2 (390–400ppm) under rainfed conditions and supplemental irrigation during three seasons, 2010–2012. Yields were significantly increased by 26% under eCO2 due to an increase in the number of pods per area. Grain size, the number of grains per pod and the harvest index remained unaffected by eCO2. Grain nitrogen concentration (N) was slightly, but significantly, decreased by eCO2, but this was not consistent across cultivars under all water regimes. The dual purpose cultivar PBA Hayman consistently maintained grain N in response to eCO2 while the response in grain N in the cultivars Sturt and PBA Twilight depended on the irrigation treatment. While there was no evidence for consistent differences in seed yield response to eCO2 for the chosen cultivars, understanding the mechanisms for why some cultivars are able to maintain N under eCO2 would allow breeding programs to develop varieties resistant to decreases in N under eCO2.