Heat stress effects on grain dry mass and quality were studied in spring wheat genotypes (Triticum aestivum L.). Three cultivars were chosen with respect to heat tolerance: Lavett (genotype 1), selected for temperate growing conditions and two CIMMYT cultivars, Ciano-79 (genotype 2) and Attila (genotype 3), respectively heat sensitive and heat tolerant varieties for warm environments. Plants were grown in pots at two different day/night temperatures (18/13 and 25/20 °C) in climate-controlled glasshouses and exposed to a 3-day heat shock during the grain-filling phase. To investigate its effect on quality and yield the following traits were included: grain dry mass, flour protein concentration, glutenin macro polymer (GMP) fraction, glutenin particle size, and particle size distribution. The magnitude in which a heat shock during grain filling affects quality and yield depends on the heat tolerance of genotypes and the preceding ambient growth temperature. As expected, the responses to heat varied between and within genotypes; on the one hand grain size and dry mass decreased, on the other hand, protein related traits were improved. In genotype 1 (Lavett) a negative correlation was found between flour protein concentration and grain dry mass. However, in genotype 3 (Attila) flour protein concentration remained quite stable under a wide range of grain dry mass in response to the treatments. Genotype 2 (Ciano-79) showed an intermediate response. These big differences in genotypic response do have a potential significant impact for maintaining quality under adverse conditions. In all genotypes the effect of heat stress was larger when plants were grown at 18/13 °C compared to 25/20 °C; especially, grain dry mass was decreased while flour protein and GMP protein fraction, GMP quantity, as well as particle size were significantly increased. The results show that genetic differences in stability of quality and yield in wheat under heat stress can be induced by a short heat-shock treatment during the grain-filling phase. It is concluded that an evaluation of quality and yield responses of wheat genotypes to short spells of heat, could considerably be improved by a heat-shock treatment during the early grain-filling phase. © 2006 Elsevier B.V. All rights reserved.