Heat stress is a major factor limiting crop yield in many agricultural regions. During the early stage of kernel development, heat stress is particularly detrimental to subsequent dry matter accumulation since it causes disruption of cell division, sugar metabolism, and starch biosynthesis in the endosperm. The effects of heat stress on protein accumulation, however, is less well understood. Therefore, the objective of this study was to determine the mechanisms by which heat stress decreases protein accumulation and alters composition of developing maize (Zea mays L.) kernels. In this study, maize ears were heat stressed for 2 and 4d at continuous 35°C starting at 5d after pollination (DAP). Endosperms were analyzed for the relative proportion of each Osborne protein solubility class fraction and for individual zein proteins. The 2- and 4-d heat stress (DHS) treatment caused a 20 and 48% reduction in kernel final dry weight, respectively, and protein content was similarly reduced. Specifically, zein content was reduced by an average of 53%, but zein composition was only mildly affected. The concentrations of glutelin and albumin plus globulin tended to increase throughout most of 4-DHS kernel development. L-[35S]-methionine incorporation in the zein fraction was delayed by the 4 DHS when compared to control and 2-DHS treated kernels. Therefore, we concluded that heat stress during early stages of endosperm development repressed zein accumulation at the synthesis level. In contrast, later in development, zein accumulation appeared to be repressed mainly by protein degradation, which appears to be a part of the natural progression of kernel development since there was no apparent significant effect of heat stress treatments on this process.