TSH secreted from the pars distalis (PD) of the pituitary gland stimulates the thyroid gland. In contrast, TSH secreted from the pars tuberalis (PT) of the pituitary gland regulates seasonal reproduction. The ontogeny of thyrotrophs and the regulatory mechanisms of TSH are apparently different between the PD and the PT. Interestingly, fish do not have an anatomically distinct PT, and the saccus vasculosus (SV) of fish is suggested to act as a seasonal sensor. Thus, it is possible that the SV is analogous to the PT. Here we examined the ontogeny of the pituitary gland and SV using rainbow trout. A histological analysis demonstrated the development of the pituitary anlage followed by that of the SV. Lhx3 and Pit-1, which are required for the development of PD thyrotrophs, clearly labeled the pituitary anlage. The common glycoprotein α-subunit (CGA) and TSH β-subunit (TSHB) genes were also detected in the pituitary anlage. In contrast, none of these genes were detected in the SV anlage. We then performed a microarray analysis and identified parvalbumin (Pvalb) as a marker for SV development. Because Pvalb expression was not detected in the pituitary anlage, no relationship was observed between the development of the SV and the pituitary gland. In contrast to embryos, Lhx3, Pit-1, CGA, and TSHB were all expressed in the adult SV. These results suggest that the morphological differentiation of SV occurs during the embryonic stage but that the functional differentiation into a seasonal sensor occurs in a later developmental stage.