In South Portugal, rising temperatures during Spring can be considered an important factor limiting wheat yields. Heat stress assumes particular importance when the wheat crop is under irrigation, where high yield potential is needed. The main objective of this study is to evaluate, under field conditions, the response of some wheat genotypes facing high temperatures during and after anthesis. Nine durum and eight bread wheat genotypes were exposed to two different sowing dates: normal and late sowing to assure high temperatures during and after anthesis, in 1997-98 and 1998-99. Grain yield and individual grain weight were significantly affected by temperature increase in 1997-98 season. Genotype x sowing date interaction was not observed indicating that selection pressure must be applied to identify genotypes with better resistance-tolerance to heat stress.