Different biometric parameters (fresh and dry weight, diameter, volume) and biological factors (membrane permeability, detoxification mechanisms and phenols content) were investigated in peach flower buds treated with the dormancy breaking agents, hydrogen cyanamide, commercially available as Dormex. Five growing stages (S-S4) were identified based on the analysis of the biometric data. The highest bud abscission percentage was recorded after 1% Dormex application at the S1 (end of January) or at the S4 (mid of March) stages as related to peach cultivar. Largest changes in membrane permeability were recorded at the same stages of growth. These changes were probably also due to fatty-acids desaturation of membrane lipids as confirmed by the gradual increase of the -3 desaturase transcript level in peach buds. The desaturating reaction produces hydrogen peroxide that inhibits the activity itself and, probably for this reason, we observed a correspondence between the expression of a peach Cat 1 gene, catalase activity and desaturase transcript levels. Total phenols content was mainly influenced by flavanols and Dormex application at the S1 reduced specially flavan-3-ols levels. Phenols play an important role in the plant stress defence mechanism and their reduction in treated buds could partly explain the high abscission percentage measured after the early (S1) Dormex application.