High temperatures during grain filling can reduce the yield of wheat and affect its grain protein concentration. The Gpc-B1 locus of wheat also affects grain protein concentration, but it is not known whether its effects interact with those of heat. The aim of this study was to investigate the effects of high temperature in lines with and without functional (high-protein) alleles at Gpc-B1. A highly replicated experiment was conducted in a glasshouse under control conditions (24/18°C, 14/10 h day/night), with half of the plants of each line or cultivar put into a heat chamber (37/27°C, 14/10 h day/night) at 15 days after anthesis for 3 days. Backcross derivatives with the Gpc-B1 introgression segment differed from their recurrent parents more than those without that segment. In some respects, the effects of the Gpc-B1 introgression were similar to those of the heat treatment: both could accelerate peduncle senescence, increase grain protein content and increase the percentage of unextractable polymeric protein. Unlike the heat treatment, Gpc-B1 did not reduce grain weight, indicating that factors that hasten senescence do not necessarily limit grain size. The presence of the Gpc-B1 segment did not exacerbate the effects of heat stress on any trait. ; Lancelot Maphosa, Nicholas C. Collins, Julian Taylor and Diane E. Mather