Antibodies (i) against porcine LH (A-pLH) and porcine FSH (A-pFSH), and (ii) against their subunits porcine LH beta, porcine FSH beta, porcine TSH beta, and rat LH alpha were used in the present study. The aim of this study was (1) to detect by the use of the immunoperoxidase technique the earliest stages of immunoreactive gonadotropic and thyrotropic cells, and (2) to study the development of the synthesis and the storage of LH, FSH and TSH in the fetal porcine anterior pituitary. The following results were obtained: (1) LH, FSH and TSH appeared between days 40 and 45 of fetal life; immunoreactive LH was found on day 40, immunoreactive TSH and FSH at day 45; thus, the first gonadotropic cells containing both LH and FSH were observed at day 45 (LH/FSH cells). (2) The alpha-subunit appeared to be present earlier than the beta-subunits or complete gonadotropic and thyrotropic hormones. (3) LH became manifest earlier than FSH. These findings showed clearly the onset of the synthesis of LH and FSH at days 40 to 45 of fetal life, and a progress until day 75, being complete at day 80 and resulting in a population of LH/FSH cells. At about day 90, the most predominant type of gonadotropic cell was the LH/FSH cell; however, some cells appeared to be exclusively LH cells.