In this study, we have tested the hypothesis that the plant hormone ethylene is a primary regulator of programmed seed desiccation, both under ideal conditions and during early transitions, triggered by abiotic stresses such as heat stress. During reproductive development, significant increases in ethylene production in heads and flag leaves from plants subjected to two days of heat stress at 38 ºC day /20 ºC night regime 10 days after pollination (DAP) followed by 25ºC day /20 ºC versus control-treated plants (20oC day 16oC night) were found for both ‘Karl 92' and 'Halberd'. However, the absolute amount of ethylene produced in the heat-susceptible ‘Karl 92' was 8 times greater than in the heat tolerant ‘Halberd'. ‘Karl 92' exhibited a shortened grain filling duration and a 30% reduction in both kernel numbers per spike and kernel weight compared to kernels from control-treated plants. No changes were found in these yield components between heat stress and control-treated plants of the heat tolerant variety ‘Halberd'. Ethylene production in embryos of ‘Karl92' and ‘Halberd' subjected to a one day 38ºC day 20ºC night heat stress at 15DAP was also examined. Under this condition, only the heat susceptible ‘Karl92' exhibited a significant increase in ethylene production during heat stress compared to embryos from control-treated plants maintained at a 20oC day 16oC night regime. Again, the heat susceptible ‘Karl92' exhibited an early transition to the seed desiccation stage and reductions in kernel weight and number per spike. Treatment of plants with the ethylene receptor inhibitor 1-MCP prior to heat stress partially blocked these effects. These results suggest that the developmental stage is an important factor for heat tolerance in wheat and that, indeed, ethylene may play an important role during the early transition to programmed seed desiccation in response to heat stress.