Hagfish represent the oldest extant craniates and are an important link between invertebrates and vertebrates. However, key elements of the reproductive system have not been elucidated in hagfish. There is new evidence from our recent studies that Atlantic hagfish may have a seasonal reproductive cycle. These data include seasonal changes in gonadotropin-releasing hormone, gonadal steroids, estradiol, and progesterone, corresponding to gonadal reproductive stages along with the putative identification of a functional corpus luteum. The corpus luteum in non-mammalian vertebrates secretes mainly progesterone thought to be involved in the retention of eggs and down regulation of vitellogenin synthesis. The most ancient vertebrate that is known to have a functional corpus luteum is the dogfish, Squalus acanthias. However, brown bodies, hypothesized to be corpora lutea, have been observed by scientists for over 100 years in the gonad of the hagfish. To date, data in support of these brown bodies acting as corpora lutea have consisted mainly of observational studies. Therefore, we examined the putative corpora lutea (post-ovulatory follicles) in hagfish by histology, electron microscopy, and production of progesterone and estradiol. Progesterone concentrations from post-ovulatory follicles were significantly higher (12 ± 1.5 pg/mg gonad tissue wet weight) compared to controls containing only gonadal tissues and oocytes (3.6 ± 1.5 pg/mg gonad tissue wet weight) (p < 0.05). Estradiol was detected in seven of the 13 samples containing only gonadal tissue with oocytes and ranged between 0.6 and 0.18 pg/mg gonad tissue wet weight and was not detected in any of the media containing only corpora lutea samples. Light and electron microscopy analysis supported that these structures were corpora lutea like structures (post-ovulatory follicles). From these results, we hypothesize that hagfish have functional corpora lutea like structures that produce progesterone. © 2006 Elsevier Inc. All rights reserved.