Abstract Many tree species in temperate zones require a period of low temperatures to overcome dormancy. When seedlings of western hemlock were preconditioned with 6 wks of mild, short days, their chilling requirement was met by 4 wks of a constant 5°C temperature, considerably less chilling than required by Douglas-fir. Seedlings not receiving this beneficial pretreatment generally required 6-8 wks of constant chilling to break bud. Even so, western hemlock had a lesser requirement than Douglas-fir for short mild days before chilling. These pretreatment effects and chilling requirement have implications for improving the nursery production of western hemlock seedings. Forest Sci. 25:485-490.