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Plasma concentrations of progesterone in non-pregnant female grey squirrels were never greater than 3·2 nmol/l and no significant differences were found between levels in anoestrous, pro-oestrous and oestrous animals. During pregnancy, plasma concentrations of progesterone increased significantly and reached a maximum level of 318 nmol/l at around day 35 of the 44 day period of gestation. After parturition, plasma concentrations of progesterone fell sharply. The corpora lutea of pregnancy began to regress in size at about day 30 of gestation, before the maximum levels of progesterone in the plasma were reached, which suggests that there is an extra-ovarian source of progesterone.

Chromatography of pregnancy plasma extracts showed that no significant amount of 5-or 5β-pregnane-3,20-dione was present and that progesterone accounted for 90% of the assay-positive material in pregnancy plasma from grey squirrels.