Winter wheat (Triticum aestivum L.) cv. Hereward was grown in the field in two double-walled polyethylene-covered tunnels within each of which a temperature gradient was superimposed on diurnal and seasonal fluctuations in temperature. The mean temperature between anthesis and harvest maturity varied from 14.3 to 18.4°C among plots within these tunnels. The CO2 concentration was controlled at different values in each tunnel; seasonal mean concentrations were 380 and 684 μmol CO2 mol−1 air. Crops were also grown outside the tunnels at ambient temperatures and CO2. Samples of seeds were harvested sequentially from each plot between anthesis and harvest maturity. Seed germination and seed survival during subsequent air-dry storage were determined for each sample. The onset of both ability to germinate and desiccation tolerance (ability to germinate after rapid desiccation to 10–15% moisture content and subsequent rehydration) coincided in all environments. Full germination capacity (> 97%, determined at 10°C) was reached 4–18 d before the end of the seed-filling phase (mass maturity) in most cases. There was little or no decline in germination capacity during subsequent seed development and maturation. Differences in seed quality were evident, however, throughout seed development and maturation when seed survival curves during subsequent storage were compared. Potential longevity in air-dry storage (assessed by the value K1 of the seed viability equation) improved consistently both before and after mass maturity. There was a significant positive relation between the rate of increase in potential longevity (dK1Idt) and temperature (the minimum temperature for seed quality development was 4.8 °C), but neither CO2 concentration nor production within the polyethylene tunnels affected this relation.