Abstract. For 4-month-old Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings under 17-h photoperiods in controlled environment conditions, flurprimidol (~t-(1-methlyethyl-a-[4-(trifluoromethoxy)phenyl]-5-pyrimidine-methanol) decreased incidence of second flushing from 81 to 54% and significantly reduced height growth in the second year following low-temperature treatment, in comparison to untreated controls. For seedlings under 15-h photoperiods, flurprimidol had only limited early effects, but after a period of bud dormancy, shoot growth was reduced to one-third that of the untreated controls. Under both photoperiods flurprimidol significantly depressed abscisic acid levels and gibberellin-like bioactivity. These results suggest that growth reduction in this conifer by flurprimidol may well involve inhibition of gibberellin biosynthesis.Late-summer bud flush is an unwanted occurrence in the production of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings. In nurseries, bud set is often induced by mild water stress (Duryea 1984). However, maintaining bud set and limiting height growth can be difficult. If applied too late in the growing season, then fertilization or even normal irrigation can stimulate second flushing (Lavender 1984). Although undercutting or top pruning has been used to combat second flushing (Duryea 1984), an even wider array of techniques for stopping late flushing and encouraging development of Abbreviations: GA, gibberellin; ABA abscisic acid; FPP, farnesyl pyrophosate; HPLC, high performance liquid chromatography; GC-EC, gas chromatography-electron capture; GC-MS, gas chromatography-mass spectrometry.