Two- to 3-month-old seedlings (10 to 12 cm tall) were grown in solution culture to which varying levels of ancymidol or dikegulac were added. Height growth of green ash (Fraxinus pennsylvanica Marsh.) at 11 days after treatment was inhibited ≈50% by 12.5 mg·liter–1 dikegulac. Growth of green ash and silver maple (Acer saccharinum L.) at 7 days was inhibited 75% to 80% by 0.125 mg·liter–1 ancymidol. Growth responses of these seedlings were linearly related to the logarithm of dikegulac and ancymidol concentrations. Chemical names used: 2,3:4,6-bis-O-(1-methylethylidene)-α-L-xylo-2-hexulofuranosonic acid) (dikegulac); α-cyclopropyl-α-(4-methoxy-phenyl)-5-pyrimidinemethanol (ancymidol).