Isothermal calorimetry was used to evaluate the effect of budbreaking agents (BBA) on metabolic rates R-q and R-CO2, metabolic efficiency (R-q/R-CO2) and specific growth rate (R-SG center dot Delta H-B) in apple flower buds during late winter. Three BBA were tested (hydrogen cyanamide, HC at 1.0% (v/v); thidiazuron (l-phenyl-3-(1,2,3-thiadiazol-5-yl)urea), TDZ at 0.3% (v/v) and mineral oil, MO at 1 % (v/v)). Water was used as a control. BBA application was performed at two R-q rates (4.8 and 5.3 mu W mg(-1) dw, corresponding to 30 and 15 days before budbreak, approximately). R-q, R-CO2, R-q/R-CO2 and R-SG center dot Delta H-B were measured during 18 days at 3 day intervals under forcing conditions. Budbreak percentage was scored at the end of the experiment. Significant differences in the calorimetric parameters (p <= 0.05) were found between BBA treatment and BBA application date. The highest R-q (11.4 mu W mg(-1) dw), R-CO2 (32.2 MMOI CO2 mg(-1) dw), R-SG center dot Delta H-B (2.8 mu W mg(-1) dw) and budbreak percentage (95%) values were found for HC treated buds when application was performed 15 days before budbreak. These samples showed the lowest R-q/R-CO2 value (368 kJ mol(-1)) confirming an increase in bud metabolic efficiency. These results indicate that HC is an effective BBA to overcome blooming problems associated with insufficient winter chilling on apples. (C) 2007 Elsevier B.V. All rights reserved.