The canopy net photosynthesis (Pn) of lettuce (Lactuca sativa L. ev.'Ambassador') was analyzed under controlled conditions simulating the winter glasshouse atmosphere. Prior to measurements the plants were grown in CO2-enriched air of 1000 μmol mol−1, at a photosynthetic photon flux density (PPFD) of 280 μmol m2 s−1 (400–700 nm) and a day/night air temperature of 16/13 °C. Short-term changes in CO2 concentration significantly changed the initial gradient of the photosynthetic response to incident PPFD. Maximum photosynthetic efficiency of the crop increased from 0·041 mol CO2 mol photons−1 (equivalent to 8·2 μg CO2 J−1 and 9·4% on an energy basis) at 350 μmol mol−1 to 0·055 mol CO2 photons−1 (10·9 μg CO2 J−1 and 12·7% on an energy basis) at 1000 μmol mol−1. Transfer from low to high CO2 also lowered the light compensation point, but did not affect dark respiration. The large response of Pn to transient changes in CO2 indicated that the lettuce canopy did not acclimate to growth in 1000 μmol CO2 mol−1, in contrast with the effect of growth in high CO2on Pn in single mature leaves reported earlier. A reduction in air temperature from 16 to 6 °C at a concentration of 1000 μmol CO2 mol−1 halved the rate of dark respiration and reduced the light compensation point, but had no direct effect on the maximum efficiency with which the crop utilized light. Subsequently at low light (below 200 μmol m−1 s−1) Pn was greater at 6 than 16 °C. Between a PPFD of 250 and 300 μmol m−2 s−1 canopy Pn was similar at all temperatures. Addition of 2·0 μmol mol−1 nitric oxide to an atmosphere of 1000 μmol CO2 mol−1 caused a rapid and reversible reduction of canopy Pn, which was greater at the lowest temperatures. The average inhibition was 6·6 at 16 °C and 28·8% at 6 °C; this was not explained by differences in the rate of pollutant uptake, which was less in the cooler conditions. The results are discussed in relation to development of optimal growing conditions for production of glasshouse lettuce at low light and low temperature during winter in the UK.