Serum samples of harbour seals kept in captivity were analysed for progesterone and oestradiol-17β. The hormone profiles obtained were used to describe a complete reproductive cycle. A clear peak in oestradiol values, indicative of ovulation, was followed by elevated concentrations of progesterone. Implantation probably occurred 3–3·5 months thereafter. Progesterone concentrations rose significantly in the last 3–4 months of gestation, whereas oestradiol concentrations gradually increased after implantation. Lactational oestrus was marked by a peak of oestradiol on average 25 days after parturition and lactation lasted 4–5 weeks. Previous pregnancy had a marked influence on the timing of oestrus; females with offspring started a new reproductive cycle about 14 days later than previously non-pregnant seals. No differences in timing of parturition between the 2 groups were observed. This was probably the result of a flexible period of delayed implantation.