SUMMARY: The rate of CO2 production by 8-cell embryos (2½ days post coitum) and blastocysts (3½ days post coitum) was measured in mice of the Q strain. The values for blastocysts resembled those obtained by Brinster (1967) in the Swiss strain, both in mean and variance; the values for 8-cell eggs differed in both respects from those obtained by Brinster. Blastocysts in delay of implantation during lactation or after ovariectomy gave off less CO2 per hr. than either 3½-day blastocysts from normal pregnancies or blastocysts from post-partum matings when the litters were removed at birth. This indicates that blastocysts enter a state of metabolic dormancy during delay of implantation. When ovariectomized pregnant mice were maintained on 0·5 mg. progesterone per day, their blastocysts produced even less CO2/hr. than those from ovariectomized animals not given exogenous hormone. The implications of this finding are considered with respect to the role of progesterone in delay of implantation.