Chilling exposure is important in apple buds to overcome dormancy. The objective of this work was to determine the effect of different chilling treatments on apple bud calorimetric responses. One-year-old Golden Delicious trees were kept in a controlled temperature chamber at 4 degrees C for up to 1600 h. At 200 h intervals, five trees were transferred to forcing conditions at 25 degrees C. Isothermal calorimetry was used to determine metabolic activity (q), respiration rate (R-CO2), respiration efficiency (q/ R-CO2) and specific growth rate (R-SG Delta(HB)) in vegetative buds. DSC was used to estimate activation energy (E-a). Calorimetric assays were performed at 0, 15 and 30 days at forcing conditions and budbreak was recorded. No significant differences (p less than or equal to 0.05) in the above respiration parameters were found between chilling treatments during cold storage and immediately when the trees were transferred to growing conditions (0 days at 25 degrees C). Also, the lowest values registered for q (2.5 mu W mg/ dw) and R-CO2 (8 mmol CO2 mg/dw) were found during the above two periods. However, after 15 and 30 days at forcing conditions, buds increased their metabolism at a slow, but significant pace, as a direct function of chilling exposure time. After 15 days under forcing treatments q increased from 2.5 to 4 mu W mg(-1) dw, R-CO2 from 9 to 13.5 mmol CO2 mg(-1) dw and R-SG Delta(HB) from 1 to 2.5. After 30 days, these responses kept increasing, q from 2.5 to 5 mu W mg(-1) dw, R-CO2 rose from 10 to 17 mmol CO2 mg(-1) dw and R-SG Delta(HB) from 2 to 3. On both periods (15 and 30 day), highest values were found after chilling exposures more than 1000 h at 4 degrees C, which coincides with an increased budbreak of 12%. No consistent pattern was observed in q/R-CO2, regardless of exposure to forcing conditions. Cumulative chilling resulted in decreasing E-a of 29.8 and 33.3 J mol(-1) K after forcing for 15 and 30 days, respectively. In general, dormant vegetative apple buds increased their calorimetric responses after forcing for 15 and 30 days, but this was significant only when chilling equaled or exceeded 1000 h. Calorimetric parameters agree with information available elsewhere on chilling requirements for this variety