Luteal regression, follicle growth and the concentration of some plasma steroids during lactation in grey seals (Halichoerus grypus).

Ovaries of lactating grey seals, which had been shot, were measured to obtain the size of each follicle and corpus luteum. Plasma samples were collected by temporarily immobilizing lactating females. The single corpus luteum regressed rapidly after pupping and circulating progesterone levels declined at parturition and remained low throughout most of lactation. A single wave of follicular growth began about the time of parturition and gave rise to a single mature follicle towards the end of lactation. This coincided with high plasma oestradiol-17 beta concentrations and behavioural oestrus. A corpus luteum inhibited follicular growth in the ipsilateral ovary. The concentration of plasma progesterone increased in some seals late in lactation, indicating that ovulation sometimes occurred before the end of lactation. This was confirmed by observation of ovaries from shot seals.