In order to investigate the ontogenesis of cell types in the pituitary gland, anterior pituitaries were collected from female foetal sheep at days 70, 100 and 130 of gestation (term = 145 days). Cells containing the common alpha-subunit and the specific beta-subunits of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH) were immunolocalized using the avidin-biotin immunoperoxidase technique. LH beta-containing cells were first detected in the foetal pituitary by day 70 of gestation. The number and intensity of staining of these LH beta cells increased by day 100 but had declined again by day 130. Immunopositive alpha-subunit and FSH beta-cells appeared by day 100 of gestation and had further increased in number and staining intensity by day 130. Cells containing TSH beta were present at day 70 and progressively increased in abundance and intensity through gestation. These data indicate that the development of LH- and FSH-containing cells in the female foetal sheep pituitary is differentially regulated during foetal life, and that in the sheep free alpha-subunit is not produced in significant amounts before the specific beta-subunits.