High temperature stress imposed during the cell division stage of maize (Zea mays L.) kernel development adversely affects growth and mature mass. However, the processes affected by heat stress are not known. The goal of this study was to determine the mechanisms by which heat stress affects early zein accumulation in maize kernels. Intact ears of growth chamber-grown plants were subjected to heat stress (continuous 35°C) for 2 or 4 d, starting at 5 d after pollination (DAP). Both the 27 kDa and cluster 1 zeins of subfamily 4 (ZSF4C1) zein mRNA steady-state levels were significantly delayed by 4 d of heat stress (DHS), but were not affected by 2 DHS. Similarly, transcription rates of both zeins were reduced in the endosperm of kernels exposed to 4 DHS treatment up to 17 DAP. The 2 DHS treatment significantly delayed endosperm endoreduplication, up to 17 DAP, whereas 4 DHS significantly repressed it. The lack of coordinate changes among mRNA steady-state levels, transcription rates, and endoreduplication during heat stress indicates that the effects of heat stress on zein transcription rates may not be directly related to alterations in endoreduplication. Instead, zein transcription is most likely affected by a delay in endosperm development.