The mouse, Mus musculus, can maintain blastocysts in embryonic diapause in the uterus while suckling young. This study used microfluorimetry to simultaneously examine glucose and pyruvate uptake by quiescent blastocysts, and at four hourly intervals after administration of the reactivating stimulus, oestradiol‐17β. Following the non‐invasive analysis of energy metabolism, blastocysts were incubated in colcemid (0.2 mg/ml), and mitotic activity determined. Mitoses and cell numbers in reactivated embryos increased significantly within 8 and 12 hours, respectively, after oestradiol‐17β administration, compared to those of diapause (control) blastocysts (0.5 ± 0.1 vs. 0.22 ± 0.03 mitoses/embryo; P < 0.05, and 141.8 ± 1.5 vs. 133.8 ± 2.4 cells/embryo; P < 0.05). Similarly, pyruvate uptake by reactivating blastocysts (9.3 ± 1.1) was significantly higher than controls (5.8 ± 0.8 pmol/embryo/hour; P < 0.05), within 4 hours of oestradiol‐17β, but by 16 hours after oestradiol‐17β administration, pyruvate uptake by reactivating blastocysts was no longer significantly different from the delayed controls. In contrast, significant differences in glucose uptake between the reactivated and control groups were not evident until 16 hours after oestradiol‐17β (reactivating, 14.9 ± 1.5; control, 10.6 ± 1.7 pmol/embryo/hour; P < 0.05). These results demonstrate that pyruvate rather than glucose could supply the additional energy required during the first 12 hours of reactivation in the mouse, but from 16 hours after injection of the reactivating stimulus oestradiol‐17β, glucose is the predominant energy source. © 1996 Wiley‐Liss, Inc.