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The rate of CO₂ production by 8-cell embryos (2 days post coitum) and blastocysts (3 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 3-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.