Peach (Prunus persica (L.) Batsch cv Jerseydawn and Jerseyglo) flower bud hardiness was studied using exotherm analysis following application of ethephon ((2-chloroethyl) phosphoric acid, 0.7mM) in October. Rehardening varied with temperature (7 or 21C), cultivar, ethephon treatment, and sampling date. Buds were more susceptible to injury in March compared to January or February. Buds rehardened more rapidly at 21C than at 7C. `Jerseyglo' rehardened more rapidly than `Jerseydawn'. Untreated buds were less hardy and also rehardened more rapidly than treated buds. Ethephon enhanced flower bud hardiness by (1) decreasing the mean low temperature exotherm of pistils, (2) increasing the number of buds which supercooled after rehardening, and (3) it decreased the rate of rehardening.