Temperature requirements for afterripening were investigated in seeds of the winter annuals, Stellaria media, Valerianella umbilicata and Phacelia purshii. Seeds were incubated during the summer dormancy period at nine temperature regimes on wet and alternately wetted and dried sand. Low temperatures (5, 10, 15, 15/6 and 20/10°C) inhibited and high temperatures (20, 25, 30/15 and 35/20°C) promoted afterripening of the seeds. Temperatures (5 and 10°C) which promote afterripening in species with a chilling requirement (e.g., summer annuals) almost completely prevented afterripening of the three winter annuals. Thus, although dormancy is an adaptation of winter annuals to their hot, dry summer habitat, the summer temperatures are required for afterripening of the seeds.