Two ammoniacal preservatives, copper-arsenic-additive (CCA) and copper-zinc-arsenic-additive (CZAA) - developed at the Eastern Forest Products Laboratory for difficult-to-penetrate wood species - were tested in a commercial treating plant on Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) plywood. The inner plies were spruce (Picea sp.) fir (Abies sp.), and hemlock (Tsuga sp.) - all can be difficult to penetrate with aqueous solutions. The treatment schedule comprised steaming for 1.5 or 2.75 h followed by a standard full-cell treatment for 1.5 or 4h (the longer schedule was used with CZAA). There was little strength loss due to treatment and no evidence of wood collapse. Cores taken from the center of the plywood faces showed: (1) the preservative had penetrated all 5 plies; (2) plywood treated with a 3.2 percent oxides solution of CAA retained 9.3 kg/m3 (0.58 pounds per cubic foot) oxides; and (3) plywood treated with a 2.2 percent oxides solution of CZAA retained 9.9 kg/m3 (0.62 pounds per cubic foot) oxides.