Seasonal fluctuations in the plasma concentrations of progesterone and oestradiol-17 beta in the female snapping turtle (Chelydra serpentina).

Plasma concentrations of progesterone and oestradiol-17 beta were determined by radioimmunoassay procedures in the sexually mature female snapping turtle at four different stages of the annual ovarian cycle. The highest level of progesterone in the peripheral plasma was detected during the luteal stage (June), whereas the concentration of oestradiol was highest (280 pg/ml) in May, immediately before ovulation.