Plasma concentrations of oestrone, progesterone and corticosteroids during late pregnancy and after parturition in the harbour seal, Phoca vitulina.

Blood samples were taken at weekly intervals during the last 2 months of pregnancy from a mature (7-year-old) harbour seal which had conceived and was kept in captivity. The concentrations of oestrone, oestrone sulphate, progesterone and glucocorticoids were determined by radioassays. The plasma levels of unconjugated oestrone were slightly greater than those of oestrone sulphate. Total oestrone declined steadily over the last month from a peak of 2.3 ng/ml at 30 days before parturition. Plasma progesterone concentrations rose to 61 ng/ml at 2 weeks before parturition and fell to about half that value by 2 days before birth of a normal male pup. Plasma glucocorticosteroids reached a peak of 164 ng/ml, also at 2 weeks, but showed only a slight decline thereafter. A higher value (392 ng/ml) was recorded 5 days after parturition.