Fall application of 2-chloroethylphosphoric acid (ethephon) is known to delay spring budbreak in peach (Prunus persica). To study seasonal variation in peach response to dormancy-breaking plant bioregulators and their possible interaction with ethylene, peach shoots were cut in the field at various intervals during endodormancy. Shoots were dipped in the dormancy-breaking bioregulators hydrogen cyanamide (H2CN2, 100 mM) or gibberellic acid (KGA3, 130 μm), alone or in combination with 1.38 mM ethephon. Treated shoots were held in beakers of either tap water or 1 mM silver thiosulfate (STS), and placed in growth chambers with potassium permanganate traps, 12/12 h photoperiods and 21/26 C temperature regimes. Dormancy-breaking efficacy (apical budbreak at 21 days) of both bioregulators increased as endodormancy progressed. At all intervals, H2CN2, broke dormancy more effectively than KGA3. The addition of ethephon to H2CN2 application prior to any CU accumulation (20 Oct) had no effect on efficacy (80% budbreak), but its addition after accumulation of ∼50 CU (8 Nov) or ∼320 CU (14 Dec) reduced subsequent budbreak to 25% and 40%, respectively. The addition of ethephon to KGA3 applications reduced budbreak both prior to (27 Oct) and after (8 Nov) initial CU accumulation. STS in the beaker solution increased both the extent (27 Oct) and the rate (14 Dec) of KGA-induced budbreak The interaction of ethylene, bioregulator type, and endodormancy regulation will be discussed.