Progesterone levels were very low or undetectable in the male, the ovariectomized female and in young females. In the adult females, there was a mean .+-. S.E.M. [standard error of the mean] level of 2.28 .+-. 0.2 ng progesterone/ml during the presumed period of blastocyst diapause (Oct. to Dec.). During the presumed period of embryonic elongation and placental attachment (Jan. and Feb.), a significant (P < 0.01) increase in plasma progesterone levels was observed (3.98 .+-. 0.32 ng/ml).