The control of progenitor cell proliferation in concert with terminal differentiation during embryonic development is poorly understood. The present paper examines this issue in the different cell lineages of the fetal mouse pituitary. Mouse fetuses were pulse-exposed to ³H-thymidine (³H-T) on a single day between embryonic day (E) 10 and E16 (prior to the onset of hormone phenotype expression) and the ³H-T labeling index of each cell type determined 3 or 4 days later (E13–19), when hormone phenotypes were detectable. In the pars tuberalis primordium, TSHβ appeared from E13. Of these cells 75.5% were labeled when ³H-T had been administered on E10. Label decreased to 40.8% when it had been incorporated on E11 and was negligible (4.2%) when it had been taken up on E12. In the pars distalis, ACTH appeared on E13, TSHβ, and PRL on E14, LHβ/FSHβ on E15 and GH on E16. When examined on E16, all these cell types were labeled for 50–60% if ³H-T had been injected on E12, but this number dropped to about 15% when ³H-T had been given on E13. Only 5–10% of the hormonal cells had taken up label when E14, 15, and 16 were the days of ³H-T administration. The decline in overall labeling index (LI) within both parts of the pituitary was significantly smaller than that in the hormone expressing cells. It is concluded that an outspoken decline in proliferation of the cells destined to become hormone-expressing cell types occurs one to several days before these hormones come to expression. In the pars distalis, this decline occurs at a common time point i.e. between E12 and E13 for each cell type. Pars tuberalis and pars distalis TSHβ cells show distinct ³H-T labeling profiles, suggesting distinct cell lineage sources for each.